feat: migrate docs to fumadocs, merge homepage into Next.js app

- Replace Mintlify with fumadocs (Next.js 16 + Tailwind v4)
- Move landing page into the same Next.js app at /
- Docs at /docs with fumadocs v16 layout and navigation
- Add llms.txt, llms-full.txt, and per-page .mdx endpoints
- Add pnpm workspace for docs
- Add changesets release workflow and GitHub Pages workflow
- Delete standalone website/ directory

Author: ericallam

docs/.gitignore

@@ -0,0 +1,4 @@
+node_modules
+.next
+out
+.source

docs/api/.gitkeep

@@ -0,0 +1,4 @@
+{
+  "title": "API",
+  "pages": ["trail", "scope", "child", "timing", "sessions"]
+}