Document rewrite_response

[ryanlovett]

Bug description

rewrite_response was added in #209 to enable proxies to dynamically alter the response to the client. This is a very cool feature and other projects like jupyter-rsession-proxy make use of it, but it is not documented.

How to reproduce

1. Go to https://jupyter-server-proxy.readthedocs.io/
2. Search for rewrite_response
3. Find nothing except for mentions in the changelog

Expected behaviour

Because I know the feature exists, I expected to find documentation for it. Others wouldn't know to search for it unless they were browsing the source code.