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/eslint.config.mjs

@@ -0,0 +1,21 @@
+import coreWebVitals from 'eslint-config-next/core-web-vitals';
+import typescript from 'eslint-config-next/typescript';
+
+/** @type {import('eslint').Linter.Config[]} */
+const config = [
+  {
+    ignores: [
+      '.next/**',
+      '.source/**',
+      'out/**',
+      'node_modules/**',
+      'next-env.d.ts',
+      // Generated API reference pages (fumadocs-openapi output)
+      'content/docs/**/reference/api/**',
+    ],
+  },
+  ...coreWebVitals,
+  ...typescript,
+];
+
+export default config;