Add documentation about how to create and use the Gradio FastAPI app (#2263)

* added a demo

* added in docs

Author: abidlabs

demo/custom_path/run.py

@@ -0,0 +1,17 @@
+from fastapi import FastAPI
+import gradio as gr
+
+CUSTOM_PATH = "/gradio"
+
+app = FastAPI()
+
+@app.get("/")
+def read_main():
+    return {"message": "This is your main app"}
+
+io = gr.Interface(lambda x: "Hello, " + x + "!", "textbox", "textbox")
+gradio_app = gr.routes.App.create_app(io)
+
+app.mount(CUSTOM_PATH, gradio_app)
+
+# Run this from the terminal as you would normally start a FastAPI app: `uvicorn run:app` and navigate to http://localhost:8000/gradio in your browser.

guides/1)getting_started/3)sharing_your_app

@@ -90,18 +90,28 @@ This will document the endpoint `/api/addition/` to the automatically generated
 
 ## Authentication
 
-You may wish to put an authentication page in front of your app to limit who can open your app. With the `auth=` keyword argument in the `launch()` method, you can pass a list of acceptable username/password tuples; or, for more complex authentication handling, you can even pass a function that takes a username and password as arguments, and returns True to allow authentication, False otherwise. Here's an example that provides password-based authentication for a single user named "admin":
+You may wish to put an authentication page in front of your app to limit who can open your app. With the `auth=` keyword argument in the `launch()` method, you can provide a tuple with a username and password, or a  list of acceptable username/password tuples;  Here's an example that provides password-based authentication for a single user named "admin":
 
 ```python
 demo.launch(auth=("admin", "pass1234"))
 ```
 
-Here's one that accepts any login where the username and password are the same.
+For more complex authentication handling, you can even pass a function that takes a username and password as arguments, and returns True to allow authentication, False otherwise. This can be used for, among other things, making requests to 3rd-party authentication services.
+
+Here's an example of a function that accepts any login where the username and password are the same:
 
 ```python
 def same_auth(username, password):
     return username == password
 demo.launch(auth=same_auth)
 ```
 
+## Mounting Within Another FastAPI App
+
+In some cases, you might have an existing FastAPI app, and you'd like to add a path for a Gradio demo. You can do this by easily using the `gradio.routes.App.create_app()` function, which creates a FastAPI app (but does not launch it), and then adding it to your existing FastAPI app with `FastAPI.mount()`.
+
+Here's a complete example:
+
+$code_custom_path
 
+Note that this approach also allows you run your Gradio apps on custom paths (`http://localhost:8000/gradio` in the example above).