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/tools/shared/output-block.tsx

@@ -0,0 +1,31 @@
+'use client';
+
+import QRCode from 'react-qr-code';
+import { CopyButton } from './copy-button';
+
+export function OutputBlock({
+  label,
+  value,
+  qr = false,
+}: {
+  label: string;
+  value: string;
+  qr?: boolean;
+}) {
+  return (
+    <div className="overflow-hidden rounded-xl border">
+      <div className="flex items-center justify-between gap-2 border-b bg-fd-muted/40 px-3 py-2">
+        <span className="text-xs font-medium text-fd-muted-foreground">{label}</span>
+        <CopyButton value={value} />
+      </div>
+      <pre dir="ltr" className="max-h-80 overflow-auto p-3 text-start text-xs leading-relaxed">
+        <code>{value}</code>
+      </pre>
+      {qr && value ? (
+        <div className="flex justify-center border-t bg-white p-4">
+          <QRCode value={value} size={180} />
+        </div>
+      ) : null}
+    </div>
+  );
+}