docs: vendor the documentation site into the monorepo

Fold the standalone 3x-ui-docs project (Next.js 16 + Fumadocs, deployed to
docs.sanaei.dev) into docs/ so the panel and its documentation share a single
source of truth, the way sing-box keeps its docs in-tree. The old repo becomes
redundant and can be retired.

- Import the full site under docs/ (app, components, content, lib, public,
  scripts, config). The self-contained pnpm project sits alongside the existing
  engineering notes with no filename collisions.
- Re-point "Edit on GitHub" links from MHSanaei/3x-ui-docs to this repo's
  docs/content/docs path (docs/lib/shared.ts, docs/app/.../page.tsx).
- Add docs-ci.yml and docs-deploy.yml under .github/workflows/, scoped to
  docs/** and run with working-directory: docs, since GitHub only runs
  workflows from the repo-root .github/. deploy-static.yml's GitHub Pages
  publish (CNAME docs.sanaei.dev) carries over unchanged.

Follow-up (outside this commit): attach the docs.sanaei.dev custom domain to
this repository's Pages (or set the Vercel project's root directory to docs),
confirm the site is live from the monorepo, then delete MHSanaei/3x-ui-docs.

docs/lib/source.ts

@@ -0,0 +1,42 @@
+import { docs } from 'collections/server';
+import { loader } from 'fumadocs-core/source';
+import { lucideIconsPlugin } from 'fumadocs-core/source/lucide-icons';
+import { openapiPlugin } from 'fumadocs-openapi/server';
+import { i18n } from '@/lib/i18n';
+import { docsContentRoute, docsImageRoute, docsRoute } from './shared';
+
+// See https://fumadocs.dev/docs/headless/source-api for more info.
+// `i18n` builds one page tree per language; untranslated pages fall back to en.
+// `openapiPlugin` adds HTTP-method badges to generated API reference pages.
+export const source = loader({
+  i18n,
+  baseUrl: docsRoute,
+  source: docs.toFumadocsSource(),
+  plugins: [lucideIconsPlugin(), openapiPlugin()],
+});
+
+export function getPageImage(page: (typeof source)['$inferPage']) {
+  const segments = [...page.slugs, 'image.png'];
+
+  return {
+    segments,
+    url: `${docsImageRoute}/${segments.join('/')}`,
+  };
+}
+
+export function getPageMarkdownUrl(page: (typeof source)['$inferPage']) {
+  const segments = [...page.slugs, 'content.md'];
+
+  return {
+    segments,
+    url: `${docsContentRoute}/${segments.join('/')}`,
+  };
+}
+
+export async function getLLMText(page: (typeof source)['$inferPage']) {
+  const processed = await page.data.getText('processed');
+
+  return `# ${page.data.title} (${page.url})
+
+${processed}`;
+}