Document the mechanics of Go

@trizzi @sabrams I wrote this document because A) Go is unlike other languages, B) the rules for versions, modules, and fetching are complex, and C) it is difficult to locate all of the relevant documentation. And I found myself frequently repeating parts of this, in issues, MRs, and comments. So I decided it was time to write it all down.

If you think it would be valuable, I would like to contribute this to GitLab's documentation. I expect GitLab will introduce additional Go-related features, and the Go proxy documentation should focus on administering and using the Go proxy. I think it would be beneficial to have separate documentation to help GitLab developers and users understand the mechanics of Go.

@stanhu If you are unfamiliar with Go or the Go proxy system, this may be useful context for some of the details of !27746 (merged).