refactor: drop the remaining docusaurus names from identifiers and comments (#589)

Follow-up to #588.

- rename `DocusaurusSidebarItem` -> `SidebarSourceItem`, `fromDocusaurus` -> `fromSidebarSource`, `docusaurusSidebars` -> `sidebarSources`
- reword comments and READMEs that described behaviour by comparison with docusaurus; fix the stale note in `sites/docs/src/config/api.ts`
- drop `.docusaurus/` from `.gitignore`

No functional change. `pnpm check` has 0 errors and `pnpm build` still builds 6690 pages.

<!-- cloudflare-preview --> Preview: https://pr-589.docs-gitea-com.pages.dev

Reviewed-on: https://gitea.com/gitea/docs/pulls/589
Reviewed-by: silverwind <[email protected]>

packages/content-loader/src/index.ts

@@ -74,7 +74,7 @@ const transformVersion = '2';
 
 /**
  * Loads the gitea documentation straight out of the content directories, which
- * keep the layout the docusaurus site used, instead of copying them into
+ * keep their original layout, instead of copying them into
  * `src/content/docs`. Every source file is mapped to its final route by the
  * product matrix, so the release and translation workflows are untouched.
  */
@@ -190,7 +190,7 @@ async function parseFile(
 
   const substituted = applyVariables(contents, segment.version.variables);
   const { frontmatter, body: rawBody } = splitFrontmatter(substituted);
-  // a translated `slug` moves the page, exactly like it did under docusaurus
+  // a `slug` moves the page
   const id = routeIdOf(segment, routeSuffix(segment, file, asString(frontmatter.slug)));
   const { title: headingTitle, body } = extractTitle(rawBody);
   const title = asString(frontmatter.title) ?? headingTitle;
@@ -289,7 +289,7 @@ async function renderPage(
   return broken;
 }
 
-/** The index page docusaurus generated for a category. */
+/** The index page generated for a category. */
 interface ParsedCategory {
   segment: Segment;
   id: string;
@@ -301,7 +301,7 @@ interface ParsedCategory {
 }
 
 /**
- * Recreates the index pages docusaurus generated from `_category_.json`, so
+ * Recreates the index pages generated from `_category_.json`, so
  * that `/category/installation/` and `/usage/actions/` keep resolving.
  */
 async function resolveCategories(