Add content on forks

[mocdaniel]

Same as in #232, training participants frequently ask about the fork functionality of GitLab.

Maybe adding a lab where they fork a preexisting internal repository on the platform and explaining the implications of a fork might make sense.

[martialblog]

We could add a "GitLab also supports forks" on this slide: "Forking Workflow with GitHub"

And or "The trainer will show it"