doc: howto configure bazel downloader with pip.parse

I think I've seen this question a few times: I want to customize where wheels are downloaded from. What parameters do I set in (pip.parse or compile_pip_requirements) to do that?

When the answer is non-obvious: use bazel downloader to redirect the url requests.

This is mentioned in the regular pypi docs, but only briefly; it's easy to overlook.

We should give an example in the docs on how to do this.

A dedicated "how to" sub-directory of docs is probably a good idea. It'll be a good place to put prose documentation that draws people in, and we can easily expand it with arbitrary topics and content without it feeling out of place or bloating the more dry/technical/reference docs.