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]>

scripts/cut-version.mjs

@@ -1,6 +1,6 @@
 #!/usr/bin/env node
 /**
- * Cuts a documentation version, the replacement for `docusaurus docs:version`.
+ * Cuts a documentation version: freezes the current docs as a released 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