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/proxy.ts

@@ -0,0 +1,13 @@
+import { createI18nMiddleware } from 'fumadocs-core/i18n/middleware';
+import { i18n } from '@/lib/i18n';
+
+// Next 16 "proxy" (middleware). The i18n middleware detects the locale and
+// rewrites `/docs/...` -> `/en/docs/...` internally (en is the hidden default),
+// while `/fa/...`, `/ru/...`, `/zh/...` keep their prefix.
+export default createI18nMiddleware(i18n);
+
+export const config = {
+  // Run on everything except API routes, Next internals, and files with an
+  // extension (og images, llms.txt, sitemap.xml, favicon, content.md, ...).
+  matcher: ['/((?!api|_next|.*\\..*).*)'],
+};