Update user documentation of Project Migration status

What does this MR do and why?

This updates the user documentation for the GitLab migration feature, with particular focus on the Project migration work.

It addresses changes requested in #350089 (closed)

How to set up and validate locally

1. pull and checkout into the branch
2. run the gdk: gdk start
3. visit
   • http://gdk.test:3000/help/user/group/import/index.md
   • http://gdk.test:3000/help/development/bulk_import.html

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.

Related to #350089 (closed)