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

* Address comments

* Lint

* Add py3 warning

python/ray/experimental/serve/examples/echo_full.py

@@ -0,0 +1,51 @@
+"""
+Full example of ray.serve module
+"""
+
+import ray
+import ray.experimental.serve as serve
+import requests
+import time
+
+# initialize ray serve system.
+# blocking=True will wait for HTTP server to be ready to serve request.
+serve.init(blocking=True)
+
+# an endpoint is associated with an http URL.
+serve.create_endpoint("my_endpoint", "/echo")
+
+
+# a backend can be a function or class.
+def echo_v1(request):
+    return request
+
+
+serve.create_backend(echo_v1, "echo:v1")
+
+# We can link an endpoint to a backend, the means all the traffic
+# goes to my_endpoint will now goes to echo:v1 backend.
+serve.link("my_endpoint", "echo:v1")
+
+print(requests.get("http://127.0.0.1:8000/echo").json())
+# The service will be reachable from http
+
+print(ray.get(serve.get_handle("my_endpoint").remote("hello")))
+
+# as well as within the ray system.
+
+
+# We can also add a new backend and split the traffic.
+def echo_v2(request):
+    # magic
+    return "something new"
+
+
+serve.create_backend(echo_v2, "echo:v2")
+
+# The two backend will now split the traffic 50%-50%.
+serve.split("my_endpoint", {"echo:v1": 0.5, "echo:v2": 0.5})
+
+# Observe requests are now split between two backends.
+for _ in range(10):
+    print(requests.get("http://127.0.0.1:8000/echo").json())
+    time.sleep(0.5)