Rebuild the site with Astro and Starlight (#496)

Replace [#307](https://gitea.com/gitea/docs/issues/307)
Closes [#237](https://gitea.com/gitea/docs/issues/237)

## Summary

Rebuilds docs.gitea.com with [Astro](https://astro.build) and [Starlight](https://starlight.astro.build), replacing Docusaurus. Every
published url keeps working and no content file was moved: the site is a new rendering layer over the existing `docs/`, `versioned_docs/`, `i18n/`, `runner-docs/` and `static/swagger-*.json` trees.

For the first time the API reference is part of the site rather than a Redoc
bundle: all seven swagger documents are rendered into real pages, one per
operation, so they are linkable, crawlable and searchable.

## Why

- the Docusaurus build needs an 8 GB heap and about 2 minutes for 2478 pages;
  the Astro build produces 5751 pages, API reference included, in about 100
  seconds
- the API reference was a single client rendered Redoc page per version, absent
  from the site search and from search engines
- sidebars, version lists and language lists were configured in three different
  places and drifted apart

## What is in here

- **`packages/content-loader`** — the product × version × language matrix
  (`products.ts`) and an Astro content loader that reads the existing markdown
  trees directly. The matrix is the single source of truth: content loading,
  sidebars, the version and language pickers, the version banner, the search
  facets and the API schemas are all derived from it.
- **Content compatibility** — frontmatter `slug`, `sidebar_position` and
  `sidebar_label` keep working, `_category_.json` still drives sidebar labels,
  order and the generated category pages, `@version@` style release variables are
  still substituted, `:::note` admonitions become Starlight asides and the 2778
  relative `*.md` links are rewritten to urls while the site is built.
- **API reference** — `starlight-openapi` renders the seven swagger documents.
  Operation urls are hyphenated (`/api/operations/list-admin-workflow-jobs/`),
  the sidebar shows the operations of the version being read with a coloured
  HTTP method badge, and the overview page links to one page per tag instead of
  repeating all 484 operations.
- **Navigation** — a version picker that follows the reader to the same page in
  another version, a language picker that only offers the languages the current
  product is published in, and product links for Docs, API, Runner and
  Enterprise.
- **Theme** — the light and dark palettes of about.gitea.com, mapped onto the
  Starlight variables.
- **Search** — Pagefind by default; Algolia DocSearch takes over when
  `PUBLIC_DOCSEARCH_APP_ID` and `PUBLIC_DOCSEARCH_API_KEY` are set. Every page
  carries `docsearch:product`, `docsearch:version` and `docsearch:language` meta
  tags, so a search stays inside what is being read.
  `cloudflare/docsearch-crawler.json` holds the crawler configuration.

## Bugs fixed along the way

- `GET /user/applications/oauth2` and `GET /user/applications/oauth2/{id}` have
  operation ids that only differ in case, so they collapsed onto the same url
  and one of the two pages was silently dropped. They are now
  `user-get-oauth2-application` and `user-get-oauth2-application-by-id`.
- The API overview and every tag page were all titled "Overview".

## Url compatibility

The routes of both builds were compared page by page during the migration with
`scripts/url-diff.mjs`:

```
identical: 2368, missing: 0, accepted: 110, added: 3292
```

The 110 accepted ones are all redirected in `cloudflare/_redirects`: the
localized copies of the English-only API and Runner docs, the Docusaurus search
page, and `/1.27/`, `/runner/3/`, `/api/1.27/` which are aliases of the versions
served at the product root. The added ones are the API operation and tag pages
plus the routes the Starlight language fallback serves in English when a
translation is missing — Docusaurus answered those with a 404.

## Workflows

- `checks` builds the site and type checks it on every pull request
- `Build and Publish Docs site` publishes `sites/docs/dist` to S3/CloudFront and
  to Cloudflare Pages, and copies `cloudflare/_headers` and
  `cloudflare/_redirects` into the deployment
- `update swagger files` and `update runner reference` are unchanged, they only
  touch content
- `make cut-version PRODUCT=docs VERSION=1.28` replaces
  `docusaurus docs:version`

## Removed

`docusaurus.config.js`, the swizzled theme under `src/`, and the Docusaurus UI
translations (`i18n/*/code.json`, `i18n/*/docusaurus-theme-classic/`). The
strings those carried — the product names, the footer column titles and the
outdated translation notice — were ported to
`sites/docs/src/config/strings.ts`; Starlight ships the rest of its interface in
both Chinese locales. The documentation content itself is untouched.

## Follow ups

- nine relative links are broken in the sources and reported by every build, the
  same ones Docusaurus warned about; `GITEA_DOCS_STRICT_LINKS=true` turns them
  into an error once they are fixed
- `cloudflare/worker.js` has to be deployed for `/enterprise/` to keep resolving
- the Algolia index has to be created and crawled before the search credentials
  are set

## Testing

```shell
make install
make serve-fast   # english, the version served at the root
make serve        # the whole matrix
make build        # 5751 pages
make check        # 0 errors
make serve-built  # build and serve, the only way to try the search locally
```

## Screenshots

<img width="1371" alt="image.png" src="attachments/69acdd77-cc89-4635-a8bd-1163a34afa86">

<img width="1810" alt="image.png" src="attachments/76212416-4825-4b4d-bf59-3e549900c96f">

<img width="1789" alt="image.png" src="attachments/7be84cda-5abe-48bd-8839-7ae1ee7806e7">

---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/docs/pulls/496
Reviewed-by: bircni <[email protected]>

scripts/cut-version.mjs

@@ -0,0 +1,94 @@
+#!/usr/bin/env node
+/**
+ * Cuts a documentation version, the replacement for `docusaurus docs:version`.
+ *
+ *   node scripts/cut-version.mjs docs 1.28     # freezes docs/ as 1.28
+ *   node scripts/cut-version.mjs runner 4      # freezes runner-docs/ as 4
+ *
+ * For the docs it copies the english tree, every translation and the sidebar
+ * file; for the runner the english tree and its sidebar. The version list
+ * (`versions.json`, `runner-docs_versions.json`) is updated too.
+ *
+ * The label, the release variables (`@version@`, `@dockerVersion@`, ...) and
+ * which version is served at the root live in
+ * `packages/content-loader/src/products.ts` and are edited by hand; the script
+ * prints a reminder.
+ */
+import { access, cp, mkdir, readFile, readdir, writeFile } from 'node:fs/promises';
+import { createRequire } from 'node:module';
+import path from 'node:path';
+import process from 'node:process';
+
+const [product, version] = process.argv.slice(2);
+
+if (!product || !version || !['docs', 'runner'].includes(product)) {
+  console.error('usage: node scripts/cut-version.mjs <docs|runner> <version>');
+  process.exit(1);
+}
+
+const root = path.resolve(import.meta.dirname, '..');
+const require = createRequire(import.meta.url);
+
+async function exists(relative) {
+  try {
+    await access(path.join(root, relative));
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/** Copies a directory, refusing to overwrite a version that was already cut. */
+async function freeze(from, to) {
+  if (await exists(to)) {
+    throw new Error(`${to} already exists, remove it first to cut the version again`);
+  }
+  await mkdir(path.join(root, path.dirname(to)), { recursive: true });
+  await cp(path.join(root, from), path.join(root, to), { recursive: true });
+  console.log(`  ${from} -> ${to}`);
+}
+
+/** Freezes a sidebar module as the json file the versioned sidebars use. */
+async function freezeSidebar(from, to) {
+  if (await exists(to)) throw new Error(`${to} already exists`);
+  const sidebar = require(path.join(root, from));
+  await writeFile(path.join(root, to), `${JSON.stringify(sidebar, null, 2)}\n`);
+  console.log(`  ${from} -> ${to}`);
+}
+
+/** Prepends the version to the list, which is ordered newest first. */
+async function addToVersions(file) {
+  const target = path.join(root, file);
+  const versions = JSON.parse(await readFile(target, 'utf-8'));
+  if (versions.includes(version)) throw new Error(`${version} is already listed in ${file}`);
+  versions.unshift(version);
+  await writeFile(target, `${JSON.stringify(versions, null, 4)}\n`);
+  console.log(`  ${file} now starts with ${version}`);
+}
+
+console.log(`cutting ${product} ${version}`);
+
+if (product === 'docs') {
+  await freeze('docs', `versioned_docs/version-${version}`);
+  await freezeSidebar('sidebars.js', `versioned_sidebars/version-${version}-sidebars.json`);
+
+  for (const locale of await readdir(path.join(root, 'i18n'))) {
+    const from = `i18n/${locale}/docusaurus-plugin-content-docs/current`;
+    if (!(await exists(from))) continue;
+    await freeze(from, `i18n/${locale}/docusaurus-plugin-content-docs/version-${version}`);
+  }
+
+  await addToVersions('versions.json');
+} else {
+  await freeze('runner-docs', `runner-docs_versioned_docs/version-${version}`);
+  await freezeSidebar(
+    'runner-sidebars.js',
+    `runner-docs_versioned_sidebars/version-${version}-sidebars.json`,
+  );
+  await addToVersions('runner-docs_versions.json');
+}
+
+console.log(
+  `\nnow edit packages/content-loader/src/products.ts: add ${version} to the ${product} versions,` +
+    ' give it a label and its release variables, and decide which version is served at the root.',
+);