Clarify GitLab provisioning groups vs. projects

Why is this change being made?

Frequently team members submit access requests for top level group accesses rather than individual project access as they may not understand fully the permissions that their Maintainer/Owner access at a top level group grants them. To adhere to our principle of least privilege we should be clear that team members should almost always receive access below the top level groups rather than at the top level.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
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  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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