docs: Add sitemap.md and link from all docs pages (#11515)

## Summary

- Adds a `sitemap.md` file to the docs for LLM-assisted navigation and
quick orientation
- Appends a link to the sitemap at the bottom of all 96 docs pages

The sitemap follows the format used by [Vercel
docs](https://vercel.com/docs/sitemap.md), including Type, Summary,
Prerequisites, and Topics metadata for each page.

## Test

Visit `/docs/sitemap.md` to view the full sitemap. Each docs page now
ends with:

```markdown
---

[View full sitemap](/docs/sitemap.md)
```

---------

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>

Author: anthonyshew

docs/site/app/sitemap.md/route.ts

@@ -0,0 +1,95 @@
+import { source } from "@/lib/geistdocs/source";
+
+export const revalidate = false;
+
+interface PageNode {
+  title: string;
+  description: string;
+  url: string;
+  children: PageNode[];
+}
+
+function buildTree(
+  pages: Array<{ url: string; data: { title: string; description?: string } }>
+): PageNode[] {
+  const root: PageNode[] = [];
+  const map = new Map<string, PageNode>();
+
+  // Sort pages by URL to ensure parents come before children
+  const sorted = [...pages].sort((a, b) => a.url.localeCompare(b.url));
+
+  for (const page of sorted) {
+    const node: PageNode = {
+      title: page.data.title,
+      description: page.data.description ?? "",
+      url: page.url,
+      children: []
+    };
+    map.set(page.url, node);
+
+    // Find parent by removing last segment
+    const segments = page.url.split("/").filter(Boolean);
+    if (segments.length <= 1) {
+      // Top-level page (e.g., /docs)
+      root.push(node);
+    } else {
+      // Try to find parent
+      const parentUrl = "/" + segments.slice(0, -1).join("/");
+      const parent = map.get(parentUrl);
+      if (parent) {
+        parent.children.push(node);
+      } else {
+        // No direct parent found, add to root
+        root.push(node);
+      }
+    }
+  }
+
+  return root;
+}
+
+function renderNode(node: PageNode, indent: number): string {
+  const prefix = "    ".repeat(indent);
+  const lines: string[] = [];
+
+  lines.push(`${prefix}- [${node.title}](${node.url})`);
+  lines.push(`${prefix}    - Summary: ${node.description}`);
+
+  for (const child of node.children) {
+    lines.push("");
+    lines.push(renderNode(child, indent + 1));
+  }
+
+  return lines.join("\n");
+}
+
+export const GET = async () => {
+  const pages = source.getPages("en");
+
+  const tree = buildTree(pages);
+
+  const header = `# Turborepo Documentation Sitemap
+
+## Purpose
+
+This file is a high-level semantic index of the documentation.
+It is intended for:
+
+- LLM-assisted navigation (ChatGPT, Claude, etc.)
+- Quick orientation for contributors
+- Identifying relevant documentation areas during development
+
+It is not intended to replace individual docs.
+
+---
+
+`;
+
+  const body = tree.map((node) => renderNode(node, 0)).join("\n\n");
+
+  return new Response(header + body, {
+    headers: {
+      "Content-Type": "text/markdown"
+    }
+  });
+};

docs/site/lib/geistdocs/source.ts

@@ -86,7 +86,11 @@ export const getLLMText = async (page: InferPageType<typeof source>) => {
 
   return `# ${page.data.title}
 
-${cleaned}`;
+${cleaned}
+
+---
+
+[View full sitemap](/sitemap.md)`;
 };
 
 // Blog loaders

docs/site/proxy.ts

@@ -43,9 +43,9 @@ const proxy = (request: NextRequest, context: NextFetchEvent) => {
 };
 
 export const config = {
-  // Matcher ignoring `/_next/`, `/api/`, static assets, favicon, feed.xml, sitemap.xml, robots.txt, schema JSON files, etc.
+  // Matcher ignoring `/_next/`, `/api/`, static assets, favicon, feed.xml, sitemap.xml, sitemap.md, robots.txt, schema JSON files, etc.
   matcher: [
-    "/((?!api|_next/static|_next/image|favicon.ico|feed.xml|sitemap.xml|robots.txt|schema\\.json|schema\\.v\\d+\\.json|microfrontends/schema\\.json).*)"
+    "/((?!api|_next/static|_next/image|favicon.ico|feed.xml|sitemap.xml|sitemap.md|robots.txt|schema\\.json|schema\\.v\\d+\\.json|microfrontends/schema\\.json).*)"
   ]
 };