Document more package index host/cache/mirror options

[EpicWink]

Update "Package index mirrors and caches" guide to include more options, and better describe classes of options.

I think the "Hosting your own simple repository" guide is very related, with a bit of overlap with this PR. I'm not sure what the path forward is here.

Also updates bandersnatch docs link #1083.

Possibly also related: #601

[EpicWink]

Also not sure whether to move this list to the Project Summaries page

[hauntsaninja]

In case it's useful, here's my simple caching proxy: https://github.com/hauntsaninja/nginx_pypi_cache I find it works significantly better than devpi

[EpicWink]

Maybe PyPI's own code base deserves a mention here? Not sure how feasible it is to self-host it.

It's already listed in the projects page

[EpicWink]

#1366 is an alternative to this PR