Update group-stage-category changes info

Cynthia "Arty" Ngrequested to merge
product-14293-changes into main

Updates the information on how to make group, stage, section, category changes.

Information integrates changes based on gitlab-com/www-gitlab-com!140522 (merged) with removal of some duplicate information.

Related to gitlab-com/Product#14293 (closed)

Edited by Cynthia "Arty" Ng

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