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/categories.ts

@@ -2,7 +2,7 @@ import { promises as fs } from 'node:fs';
 import path from 'node:path';
 import type { Segment } from './segments.js';
 
-/** `_category_.json`, the docusaurus per directory sidebar metadata. */
+/** `_category_.json`, the per directory sidebar metadata. */
 export interface CategoryMeta {
   label?: string;
   position?: number;
@@ -41,7 +41,7 @@ export function slugifyLabel(label: string): string {
 }
 
 /**
- * Route id of the index page docusaurus generates for a category.
+ * Route id of the index page generated for a category.
  *
  * Top level categories are declared by `sidebars.js` without a slug and end up
  * under `/category/<label>`; nested ones keep the slug of their