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.

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

packages/content-loader/src/products.ts

@@ -60,7 +60,7 @@ export interface ProductDef {
   /** Languages the product is published in, in display order. */
   locales: LocaleId[];
   versions: VersionDef[];
-  /** Recreate the docusaurus `_category_.json` index pages for the product. */
+  /** Recreate the `_category_.json` index pages for the product. */
   categoryPages?: boolean;
   /** Set for products served by another deployment (enterprise). */
   externalBaseUrl?: string;