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/package.json

@@ -0,0 +1,52 @@
+{
+  "name": "3x-ui-docs",
+  "version": "1.0.0",
+  "private": true,
+  "license": "GPL-3.0-or-later",
+  "type": "module",
+  "scripts": {
+    "dev": "next dev",
+    "build": "next build",
+    "start": "next start",
+    "postinstall": "fumadocs-mdx",
+    "gen:api": "node --experimental-strip-types scripts/gen-openapi.ts",
+    "typecheck": "fumadocs-mdx && next typegen && tsc --noEmit",
+    "lint": "eslint .",
+    "format": "prettier --write .",
+    "format:check": "prettier --check .",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "dependencies": {
+    "@orama/orama": "^3.1.18",
+    "fumadocs-core": "^16.10.5",
+    "fumadocs-docgen": "^3.0.10",
+    "fumadocs-mdx": "^15.0.12",
+    "fumadocs-openapi": "^11.0.5",
+    "fumadocs-ui": "^16.10.5",
+    "lucide-react": "^1.21.0",
+    "mermaid": "^11.16.0",
+    "next": "16.2.9",
+    "next-themes": "^0.4.6",
+    "react": "^19.2.7",
+    "react-dom": "^19.2.7",
+    "react-qr-code": "^2.2.0",
+    "tailwind-merge": "^3.6.0",
+    "zod": "^4.4.3"
+  },
+  "devDependencies": {
+    "@tailwindcss/postcss": "^4.3.1",
+    "@types/mdx": "^2.0.14",
+    "@types/node": "^26.0.1",
+    "@types/react": "^19.2.17",
+    "@types/react-dom": "^19.2.3",
+    "eslint": "^9.39.4",
+    "eslint-config-next": "16.2.9",
+    "postcss": "^8.5.15",
+    "prettier": "^3.8.4",
+    "tailwindcss": "^4.3.1",
+    "typescript": "^6.0.3",
+    "vitest": "^4.1.9"
+  },
+  "packageManager": "pnpm@11.9.0"
+}