Add holistic documentation on how to configure Athens to proxy private repositories

**Is your feature request related to a problem? Please describe.**

We have various documents that you have to piece together to figure out how to configure Athens to proxy to private repositories:

- https://docs.gomods.io/configuration/authentication/
- https://docs.gomods.io/configuration/filter/ (see https://github.com/gomods/athens/issues/1501 for another planned update for this)
- https://docs.gomods.io/configuration/download/

There is no single place you can go to learn about all the configuration you need to set in order to get Athens to proxy private code.

**Describe the solution you'd like**

I'd like us to add a single place you can go to learn how to configure Athens to proxy private code.

**Describe alternatives you've considered**

N/A

**Additional context**

N/A

Review the authentication, filter, and download configuration pages linked in the issue, along with issue #1501. Identify the settings needed to proxy private repositories and organize them into one discoverable guide. Done means a reader can configure private-repository proxying from the new documentation without piecing together the existing pages.

Written by the indexing model from the issue text.