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/CONTRIBUTING.md

@@ -0,0 +1,34 @@
+# Contributing to 3x-ui-docs
+
+Thanks for helping improve the 3x-ui documentation and product site!
+
+## Prerequisites
+
+This project uses **[pnpm](https://pnpm.io)** (not npm — `package-lock.json` is
+gitignored). Install dependencies and start the dev server:
+
+```bash
+pnpm install
+pnpm dev        # http://localhost:3000
+```
+
+## Scripts
+
+| Script           | Description                                           |
+| ---------------- | ----------------------------------------------------- |
+| `pnpm dev`       | Start the dev server                                  |
+| `pnpm build`     | Production build                                      |
+| `pnpm start`     | Serve the production build                            |
+| `pnpm typecheck` | Generate MDX/route types and run `tsc --noEmit`       |
+| `pnpm lint`      | ESLint (flat config)                                  |
+| `pnpm format`    | Format with Prettier                                  |
+| `pnpm test`      | Run unit tests (Vitest) for `lib/xray/*` pure logic   |
+| `pnpm gen:api`   | Generate the API reference from `public/openapi.json` |
+
+Before opening a pull request, please run `pnpm typecheck`, `pnpm lint`, and
+`pnpm test` — these are the same checks that CI runs on every PR.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the
+project's [GPL-3.0](./LICENSE) license.