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/copy-button.tsx

@@ -0,0 +1,46 @@
+'use client';
+
+import { useState } from 'react';
+import { Check, Copy } from 'lucide-react';
+import { cn } from '@/lib/cn';
+
+export function CopyButton({
+  value,
+  label = 'Copy',
+  className,
+}: {
+  value: string;
+  label?: string;
+  className?: string;
+}) {
+  const [copied, setCopied] = useState(false);
+
+  async function copy() {
+    try {
+      await navigator.clipboard.writeText(value);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    } catch {
+      // Clipboard unavailable (insecure context) — ignore.
+    }
+  }
+
+  return (
+    <button
+      type="button"
+      onClick={copy}
+      aria-label={copied ? 'Copied' : label}
+      className={cn(
+        'inline-flex items-center gap-1.5 rounded-lg border px-2.5 py-1.5 text-xs font-medium transition-colors hover:bg-fd-accent hover:text-fd-accent-foreground focus-visible:outline-2 focus-visible:outline-fd-ring',
+        className,
+      )}
+    >
+      {copied ? (
+        <Check className="size-3.5 text-brand" aria-hidden />
+      ) : (
+        <Copy className="size-3.5" aria-hidden />
+      )}
+      <span>{copied ? 'Copied' : label}</span>
+    </button>
+  );
+}