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/xray/reverse-proxy.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect } from 'vitest';
+import { buildProxyConfig, buildCertCommand, type ReverseProxyOptions } from './reverse-proxy';
+
+const base: ReverseProxyOptions = {
+  server: 'nginx',
+  domain: 'panel.example.com',
+  panelPort: '2053',
+  panelPath: '/panel',
+  certTool: 'certbot',
+};
+
+describe('buildProxyConfig — nginx', () => {
+  it('includes WebSocket upgrade headers and the upstream port', () => {
+    const cfg = buildProxyConfig(base);
+    expect(cfg).toContain('server_name panel.example.com;');
+    expect(cfg).toContain('proxy_pass http://127.0.0.1:2053;');
+    expect(cfg).toContain('proxy_set_header Upgrade $http_upgrade;');
+    expect(cfg).toContain('proxy_set_header Connection "upgrade";');
+    expect(cfg).toContain('location /panel/');
+  });
+});
+
+describe('buildProxyConfig — caddy', () => {
+  it('produces a path-scoped reverse_proxy', () => {
+    const cfg = buildProxyConfig({ ...base, server: 'caddy' });
+    expect(cfg).toContain('panel.example.com {');
+    expect(cfg).toContain('reverse_proxy /panel/* 127.0.0.1:2053');
+  });
+
+  it('proxies the whole site when path is root', () => {
+    const cfg = buildProxyConfig({ ...base, server: 'caddy', panelPath: '/' });
+    expect(cfg).toContain('reverse_proxy 127.0.0.1:2053');
+  });
+});
+
+describe('buildCertCommand', () => {
+  it('supports certbot and acme.sh', () => {
+    expect(buildCertCommand(base)).toContain('certbot certonly --nginx -d panel.example.com');
+    expect(buildCertCommand({ ...base, certTool: 'acme.sh' })).toContain(
+      'acme.sh --issue -d panel.example.com',
+    );
+  });
+});