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/app/og/docs/[...slug]/route.tsx

@@ -0,0 +1,29 @@
+import { getPageImage, source } from '@/lib/source';
+import { notFound } from 'next/navigation';
+import { ImageResponse } from 'next/og';
+import { generate as DefaultImage } from 'fumadocs-ui/og';
+import { appName } from '@/lib/shared';
+
+export const dynamic = 'force-static';
+export const revalidate = false;
+
+export async function GET(_req: Request, { params }: RouteContext<'/og/docs/[...slug]'>) {
+  const { slug } = await params;
+  const page = source.getPage(slug.slice(0, -1));
+  if (!page) notFound();
+
+  return new ImageResponse(
+    <DefaultImage title={page.data.title} description={page.data.description} site={appName} />,
+    {
+      width: 1200,
+      height: 630,
+    },
+  );
+}
+
+export function generateStaticParams() {
+  return source.getPages().map((page) => ({
+    lang: page.locale,
+    slug: getPageImage(page).segments,
+  }));
+}