Add marimo notebook mounting docs and example

- Document mounting marimo ASGI apps in the feature tour
- Add examples/marimo_mount.py showing the integration
- Verified working: marimo.create_asgi_app() mounts cleanly via api.mount()

Fixes #580.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

examples/marimo_mount.py

@@ -0,0 +1,31 @@
+"""Mount marimo notebooks inside a Responder API.
+
+Requirements:
+    pip install responder marimo
+
+Usage:
+    python examples/marimo_mount.py
+
+Then visit:
+    http://127.0.0.1:5042/hello       → Responder JSON endpoint
+    http://127.0.0.1:5042/notebooks/  → Interactive marimo notebook
+"""
+
+import marimo
+
+import responder
+
+api = responder.API()
+
+
+@api.route("/hello")
+def hello(req, resp):
+    resp.media = {"message": "Hello from Responder!"}
+
+
+# Mount marimo notebooks at /notebooks
+server = marimo.create_asgi_app().with_app(path="", root="notebooks/hello.py")
+api.mount("/notebooks", server.build())
+
+if __name__ == "__main__":
+    api.run()