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/app/sitemap.ts

@@ -0,0 +1,33 @@
+import type { MetadataRoute } from 'next';
+import { source } from '@/lib/source';
+import { i18n } from '@/lib/i18n';
+import { siteUrl } from '@/lib/shared';
+
+// Required for `output: 'export'`.
+export const dynamic = 'force-static';
+
+// Locale home pages + the canonical (English) docs pages. Other locales
+// currently fall back to English content, so we don't list them separately
+// to avoid duplicate-content entries until real translations exist.
+export default function sitemap(): MetadataRoute.Sitemap {
+  const entries: MetadataRoute.Sitemap = [];
+
+  for (const lang of i18n.languages) {
+    const prefix = lang === 'en' ? '' : `/${lang}`;
+    entries.push({
+      url: `${siteUrl}${prefix}` || siteUrl,
+      changeFrequency: 'weekly',
+      priority: 1,
+    });
+  }
+
+  for (const page of source.getPages('en')) {
+    entries.push({
+      url: `${siteUrl}${page.url}`,
+      changeFrequency: 'weekly',
+      priority: 0.8,
+    });
+  }
+
+  return entries;
+}