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/logo.tsx

@@ -0,0 +1,15 @@
+import { cn } from '@/lib/cn';
+
+// Official 3x-ui logo (media/3x-ui-{light,dark}.png from the upstream repo).
+// Theme-aware via Tailwind's `dark:` variant. Pass a height class (e.g. `h-6`);
+// width scales automatically (the artwork is 2:1).
+export function Logo({ className }: { className?: string }) {
+  return (
+    <>
+      {/* eslint-disable-next-line @next/next/no-img-element */}
+      <img src="/logo-light.png" alt="3x-ui" className={cn('w-auto dark:hidden', className)} />
+      {/* eslint-disable-next-line @next/next/no-img-element */}
+      <img src="/logo-dark.png" alt="3x-ui" className={cn('hidden w-auto dark:block', className)} />
+    </>
+  );
+}