doc: Migrate off a self-managed GitLab instance to gitlab.com

What does this MR do?

This MR is based off the request by @mbruemmer in #330492. It is a guide on migrating off a self-managed Gitlab instance to gitlab.com including checklists and some code for automation.

The wording might currently not be suitable for merging as it is written in 'story-telling' mode from a company who actually did the migration. The documentation is a verbatim copy of https://200ok.ch/posts/2021-05-04_migrate_off_a_selfmanaged_gitlab_instance_to_gitlab.com.html

@ngaskill Please feel free to take over and remove and reword whatever is necessary to bring this into a format that is suitable to be documentation from GitLab for GitLab users.

I did none of the quality requirements of an upstream GitLab MR so far. I'm solely providing the original Markdown for https://200ok.ch/posts/2021-05-04_migrate_off_a_selfmanaged_gitlab_instance_to_gitlab.com.html as discussed in #330492.

@mbruemmer mentioned that GitLab might not be able to take over the scripts as they cannot be maintained by GitLab itself. If that is the case, I could offer to make a public project on gitlab.com/200ok that could be linked to. On the one hand we don't plan on updating these scripts ourselves in the future, but on the other hand, I'd be happy to take in MRs and do some easy sanity checking before merging it in.