We should move the .netrc Auth Documentation

[arschles]

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

The "Authenticate private repositories via .netrc" documentation is at https://docs.gomods.io/design/proxy/, but that's not a good fit because it's not related to high level design.

Describe the solution you'd like

I think we should move this information to the pages under https://docs.gomods.io/install/

Describe alternatives you've considered

None

Additional context

We talked about this in the October 4, 2018 Dev Meeting

[fedepaol]

Taking this one