Add developer doc on groups and projects consolidation

What does this MR do and why?

This MR adds developer docs on the projects to project namespace transition as requested by PM at gitlab-org/manage/general-discussion#17406 (comment 699720257)

This MR is to be filled in with more details and specific good/bad examples especially as we move to Phases 2 and 3.

Related to #344456 (closed) for line editing this page after publication.

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