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/content/docs/en/help/faq.mdx

@@ -0,0 +1,47 @@
+---
+title: FAQ
+description: Frequently asked questions about 3x-ui — licensing, supported systems, databases, and clients.
+icon: CircleQuestionMark
+---
+
+## Is 3x-ui free and open source?
+
+Yes. 3x-ui is open source under the GPL-3.0 license. The source is on
+[GitHub](https://github.com/MHSanaei/3x-ui).
+
+## What systems does it run on?
+
+Most major Linux distributions (Ubuntu, Debian, CentOS/RHEL and derivatives,
+Fedora, Arch, Alpine, and more) across `amd64`, `arm64`, and other architectures.
+It also runs as a Docker container. See [Installation](/docs/guide/installation).
+
+## SQLite or PostgreSQL?
+
+SQLite is the default and is fine for most deployments. PostgreSQL is available
+for larger setups via `XUI_DB_TYPE=postgres` and `XUI_DB_DSN` — see the
+[environment variables](/docs/reference/env-vars).
+
+## Which client apps work with it?
+
+Any Xray-compatible client — for example v2rayNG, Hiddify, and Clash/Mihomo.
+Import a client's share link or QR code, or use a
+[subscription](/docs/config/subscription).
+
+## How is 3x-ui different from x-ui?
+
+3x-ui is an enhanced fork of the original X-UI project. It adds broader protocol
+support, improved stability, per-client traffic accounting, multi-node
+management, a 13-language UI, and many quality-of-life features.
+
+## How do I update?
+
+Re-run the install script (it updates in place), or pull the new Docker image.
+See [Installation](/docs/guide/installation) and the
+[releases page](https://github.com/MHSanaei/3x-ui/releases).
+
+## Where can I get help?
+
+Join the official Telegram channel [@XrayUI](https://t.me/XrayUI) for
+announcements and community support, or open an issue on
+[GitHub](https://github.com/MHSanaei/3x-ui/issues). For common problems, start
+with [Troubleshooting](/docs/help/troubleshooting).