Add Open API prefix route - correct docs behind reverse proxy (#26)

Add Open API prefix route - correct docs behind reverse proxy.

docs/tutorial/sub-applications-proxy.md

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+There are at least two situations where you could need to create your **FastAPI** application using some specific paths.
+
+But then you need to set them up to be served with a path prefix.
+
+It could happen if you have a:
+
+* **Proxy** server.
+* You are "**mounting**" a FastAPI application inside another FastAPI application (or inside another ASGI application, like Starlette).
+
+## Proxy
+
+Having a proxy in this case means that you could declare a path at `/app`, but then, you could need to add a layer on top (the Proxy) that would put your **FastAPI** application under a path like `/api/v1`.
+
+In this case, the original path `/app` will actually be served at `/api/v1/app`.
+
+Even though your application "thinks" it is serving at `/app`.
+
+And the Proxy could be re-writing the path "on the fly" to keep your application convinced that it is serving at `/app`.
+
+Up to here, everything would work as normally.
+
+But then, when you open the integrated docs, they would expect to get the OpenAPI schema at `/openapi.json`, instead of `/api/v1/openapi.json`.
+
+So, the frontend (that runs in the browser) would try to reach `/openapi.json` and wouldn't be able to get the OpenAPI schema.
+
+So, it's needed that the frontend looks for the OpenAPI schema at `/api/v1/openapi.json`.
+
+And it's also needed that the returned JSON OpenAPI schema has the defined path at `/api/v1/app` (behind the proxy) instead of `/app`.
+
+---
+
+For these cases, you can declare an `openapi_prefix` parameter in your `FastAPI` application.
+
+See the section below, about "mounting", for an example.
+
+
+## Mounting a **FastAPI** application
+
+"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.
+
+You could want to do this if you have several "independent" applications that you want to separate, having their own independent OpenAPI schema and user interfaces.
+
+### Top-level application
+
+First, create the main, top-level, **FastAPI** application, and its path operations:
+
+```Python hl_lines="3 6 7 8"
+{!./src/sub_applications/tutorial001.py!}
+```
+
+### Sub-application
+
+Then, create your sub-application, and its path operations.
+
+This sub-application is just another standard FastAPI application, but this is the one that will be "mounted".
+
+When creating the sub-application, use the parameter `openapi_prefix`. In this case, with a prefix of `/subapi`:
+
+```Python hl_lines="11 14 15 16"
+{!./src/sub_applications/tutorial001.py!}
+```
+
+### Mount the sub-application
+
+In your top-level application, `app`, mount the sub-application, `subapi`.
+
+Here you need to make sure you use the same path that you used for the `openapi_prefix`, in this case, `/subapi`:
+
+```Python hl_lines="11 19"
+{!./src/sub_applications/tutorial001.py!}
+```
+
+## Check the automatic API docs
+
+Now, run `uvicorn`, if your file is at `main.py`, it would be:
+
+```bash
+uvicorn main:app --debug
+```
+
+And open the docs at <a href="http://127.0.0.1:8000/docs" target="_blank">http://127.0.0.1:8000/docs</a>.
+
+You will see the automatic API docs for the main app, including only its own paths:
+
+<img src="/img/tutorial/sub-applications/image01.png">
+
+
+And then, open the docs for the sub-application, at <a href="http://127.0.0.1:8000/subapi/docs" target="_blank">http://127.0.0.1:8000/subapi/docs</a>.
+
+You will see the automatic API docs for the sub-application, including only its own sub-paths, with their correct prefix:
+
+<img src="/img/tutorial/sub-applications/image02.png">
+
+
+If you try interacting with any of the two user interfaces, they will work, because the browser will be able to talk to the correct path (or sub-path).