Update documentation page on migrating groups

What does this MR do and why?

This MR is towards solving #385000 (closed).

• refrain from using GitLab Migration as name
• use this page as the main page for talking about migrating GitLab groups, say that there are two ways to do that: by importing an export file, by direct transfer. Write what are the differences between them (user mapping, singular vs in bulk)
• have both those ways documented on the same page OR create subpages
• make clear that projects are migrated with groups, see Make clear that projects are migrated with groups (#383784 - closed)
• add relevant info like limits (PS document)
• link to dev docs (they need improving too)
• check if need to add sth in relation to https://docs.gitlab.com/ee/development/documentation/feature_flags.html.