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/components/home/language-switcher.tsx

@@ -0,0 +1,45 @@
+import Link from 'next/link';
+import { Languages } from 'lucide-react';
+import { i18n, locales } from '@/lib/i18n';
+import { cn } from '@/lib/cn';
+
+// Home-navbar language switcher.
+//
+// fumadocs' built-in popover switcher (`LanguageSelect`) has its item clicks
+// swallowed when it is nested inside HomeLayout's Radix `NavigationMenu` — the
+// dropdown opens but selecting a locale never fires `onChange`/`router.push`.
+// The docs sidebar isn't wrapped in a NavigationMenu, so the built-in one works
+// there and is kept. Here we use a native `<details>` toggle + real `<Link>`
+// anchors, which navigate reliably inside the navbar (like the other nav links).
+//
+// The home navbar only renders on the landing page, so the targets are simply
+// each locale's home (`/`, `/fa`, `/ru`, `/zh`).
+export function HomeLanguageSwitcher({ current }: { current: string }) {
+  return (
+    <details className="group relative [&>summary::-webkit-details-marker]:hidden">
+      <summary
+        aria-label="Choose a language"
+        className="flex cursor-pointer list-none items-center rounded-lg p-1.5 text-fd-muted-foreground transition-colors hover:bg-fd-accent hover:text-fd-accent-foreground group-open:bg-fd-accent"
+      >
+        <Languages className="size-5" />
+      </summary>
+      <div className="absolute end-0 z-50 mt-1.5 flex min-w-40 flex-col gap-0.5 rounded-lg border bg-fd-popover p-1 text-fd-popover-foreground shadow-lg">
+        <p className="p-2 text-xs font-medium text-fd-muted-foreground">Choose a language</p>
+        {locales.map(({ locale, name }) => (
+          <Link
+            key={locale}
+            href={locale === i18n.defaultLanguage ? '/' : `/${locale}`}
+            className={cn(
+              'rounded-md px-2 py-1.5 text-start text-sm transition-colors',
+              locale === current
+                ? 'bg-fd-primary/10 text-fd-primary'
+                : 'text-fd-muted-foreground hover:bg-fd-accent hover:text-fd-accent-foreground',
+            )}
+          >
+            {name}
+          </Link>
+        ))}
+      </div>
+    </details>
+  );
+}