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/mdx/mermaid.tsx

@@ -0,0 +1,44 @@
+'use client';
+
+import { useEffect, useId, useState } from 'react';
+import { useTheme } from 'next-themes';
+
+// Client-side, theme-aware Mermaid renderer. Mermaid is imported dynamically so
+// it stays out of the initial bundle and only loads on pages that use a diagram.
+export function Mermaid({ chart }: { chart: string }) {
+  const rawId = useId();
+  const id = `mmd-${rawId.replace(/[^a-zA-Z0-9]/g, '')}`;
+  const { resolvedTheme } = useTheme();
+  const [svg, setSvg] = useState('');
+
+  useEffect(() => {
+    let active = true;
+    void (async () => {
+      const mermaid = (await import('mermaid')).default;
+      mermaid.initialize({
+        startOnLoad: false,
+        securityLevel: 'strict',
+        theme: resolvedTheme === 'dark' ? 'dark' : 'default',
+        fontFamily: 'inherit',
+      });
+      try {
+        const { svg } = await mermaid.render(id, chart.trim());
+        if (active) setSvg(svg);
+      } catch {
+        if (active) setSvg('');
+      }
+    })();
+    return () => {
+      active = false;
+    };
+  }, [chart, resolvedTheme, id]);
+
+  return (
+    <div
+      className="my-6 flex justify-center overflow-x-auto rounded-xl border bg-fd-card p-4 [&_svg]:max-w-full"
+      role="img"
+      aria-label="Architecture diagram"
+      dangerouslySetInnerHTML={{ __html: svg }}
+    />
+  );
+}