Follow-up from "feat: extract gitlab stub server in a separate package" - add documentation

The following discussion from !673 (merged) should be addressed:

• @jaime started a discussion: (+3 comments)

  This is great for quick development! I think we should document this in doc/development.md so that others can use it as well.

  As a small suggestion, this main file should live under a cmd. Something like test/gitlabstub/cmd/server/main.go, WDYT?