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/features.tsx

@@ -0,0 +1,49 @@
+import {
+  Boxes,
+  Network,
+  Send,
+  ShieldCheck,
+  TerminalSquare,
+  Users,
+  type LucideIcon,
+} from 'lucide-react';
+
+// Icons map by position to the localized feature items in lib/site-i18n.ts
+// (Every major protocol, REALITY, Clients, Multi-node, Telegram, Self-hosted).
+const ICONS: LucideIcon[] = [Boxes, ShieldCheck, Users, Network, Send, TerminalSquare];
+
+export function Features({
+  heading,
+  subtitle,
+  items,
+}: {
+  heading: string;
+  subtitle: string;
+  items: { title: string; description: string }[];
+}) {
+  return (
+    <section className="mx-auto w-full max-w-6xl px-4 py-16 sm:py-24">
+      <div className="mx-auto max-w-2xl text-center">
+        <h2 className="text-2xl font-bold tracking-tight sm:text-3xl">{heading}</h2>
+        <p className="mt-3 text-fd-muted-foreground">{subtitle}</p>
+      </div>
+      <div className="mt-12 grid grid-cols-1 gap-4 sm:grid-cols-2 lg:grid-cols-3">
+        {items.map(({ title, description }, i) => {
+          const Icon = ICONS[i] ?? Boxes;
+          return (
+            <div
+              key={title}
+              className="rounded-2xl border bg-fd-card p-6 transition-colors hover:border-fd-primary/40"
+            >
+              <div className="inline-flex size-11 items-center justify-center rounded-xl bg-brand/10 text-brand">
+                <Icon className="size-6" aria-hidden />
+              </div>
+              <h3 className="mt-4 font-semibold">{title}</h3>
+              <p className="mt-2 text-sm text-fd-muted-foreground">{description}</p>
+            </div>
+          );
+        })}
+      </div>
+    </section>
+  );
+}