Write "GitLab is in production" transition doc

Let's document on our website (so we can improve it thanks to the feedback of early adopters):

• How to activate your GitLab user account (blocked by #70 (closed))
• How to update your Git config
• Point to our new GitLab doc
• Point to other major doc pages and processes that were significantly changed
• Ask about #64 (closed)

@sajolida-guest suggested a blueprint but our favorite wiki spammers will keep reverting stuff there, so I'll instead push this under contribute/working_together/GitLab.