Improve Geo documentation
What does this MR do?
Closes gitlab-org/gitlab-ee#4921,gitlab-org/gitlab-ee#4889,gitlab-org/gitlab-ee#4883,gitlab-org/gitlab-ee#4892,gitlab-org/gitlab-ee#4882,gitlab-org/gitlab-ee#4886,gitlab-org/gitlab-ee#4890,gitlab-org/gitlab-ee#4482
I've also moved every external link or link to another documentation file into the [something]: document.md
markup so we can fix and keep track easily of the links when things move around.
Edited by 🤖 GitLab Bot 🤖