Update Documentation to add 'read_repository' scope requirement for project migration

The following discussion from !70326 (merged) should be addressed:

• @georgekoltsov started a discussion:

  Additionally, this creates a requirement for provided api token to now have 2 scopes instead of 1. We now need to have api & read_repository scopes. Because of that, we need to document this change. I don't think we currently specify any required scope in our documentation https://docs.gitlab.com/ee/user/group/import/

  Because project migration is behind feature flag, we don't need to update the documentation now. May I ask you to create a follow up to update docs to reflect on the new required scope?