[Serve] First iteration of the serve doc (#5834)

* Address comments

* Lint

* Add py3 warning

doc/source/index.rst

@@ -252,6 +252,7 @@ Getting Involved
    projects.rst
    signals.rst
    async_api.rst
+   serve.rst
 
 .. toctree::
    :maxdepth: -1

doc/source/serve.rst

@@ -0,0 +1,20 @@
+Ray Serve (Experimental)
+========================
+
+Ray Serve is a serving library that exposes python function/classes to HTTP.
+It has built-in support for flexible traffic policy. This means you can easy
+split incoming traffic to multiple implementations. 
+
+With Ray Serve, you can deploy your services at any scale.
+
+.. warning::
+  Ray Serve is Python 3 only.
+
+Quickstart
+----------
+.. literalinclude:: ../../python/ray/experimental/serve/examples/echo_full.py
+
+API
+---
+.. automodule:: ray.experimental.serve
+    :members: