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/lib/layout.shared.tsx

@@ -0,0 +1,60 @@
+import type { BaseLayoutProps } from 'fumadocs-ui/layouts/shared';
+import { Heart } from 'lucide-react';
+import { Logo } from '@/components/logo';
+import { TelegramIcon } from '@/components/icons';
+import { appName, productRepoUrl, telegramChannel, telegramChannelUrl, donateUrl } from './shared';
+import { getSiteMessages } from './site-i18n';
+
+// Build locale-aware shared layout options. With `hideLocale: 'default-locale'`,
+// English URLs have no prefix while other locales are prefixed (`/fa`, `/ru`, `/zh`).
+export function baseOptions(lang: string): BaseLayoutProps {
+  const prefix = lang === 'en' ? '' : `/${lang}`;
+  const m = getSiteMessages(lang);
+
+  return {
+    nav: {
+      title: (
+        <span className="inline-flex items-center gap-2 font-semibold">
+          <Logo className="h-6" />
+          {appName}
+        </span>
+      ),
+      url: `${prefix}/`,
+    },
+    githubUrl: productRepoUrl,
+    links: [
+      {
+        text: m.documentation,
+        url: `${prefix}/docs`,
+        active: 'nested-url',
+      },
+      {
+        type: 'icon',
+        label: `Telegram channel (@${telegramChannel})`,
+        icon: <TelegramIcon />,
+        text: 'Telegram',
+        url: telegramChannelUrl,
+        external: true,
+      },
+      // Compact heart icon in the top nav bars (home + docs).
+      {
+        type: 'icon',
+        on: 'nav',
+        label: m.donate,
+        icon: <Heart />,
+        text: m.donate,
+        url: donateUrl,
+        external: true,
+      },
+      // Prominent labelled entry in the docs sidebar / mobile menu.
+      {
+        type: 'main',
+        on: 'menu',
+        icon: <Heart />,
+        text: m.donate,
+        url: donateUrl,
+        external: true,
+      },
+    ],
+  };
+}