Add documentation for private go packages

GitLab projects can be used as go packages, but in order to use a private project a .netrc file needs to be used for authentication. This is really by curl feature used by go get but we should document this somewhere.

https://ec.haxx.se/usingcurl-netrc.html

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/issues/45055

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation blurb

• Doc title

• Feature overview/description

• Feature use cases

Developer

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template