Build SSOT for account and limit settings

What does this MR do?

In #347004 (closed) Marcel identified duplication on data about repository limits:

• GitLab.com: https://docs.gitlab.com/ee/user/gitlab_com/#account-and-limit-settings
• Self-managed: https://docs.gitlab.com/ee/user/admin_area/settings/account_and_limit_settings.html (various subheadings)

Goals:

1. Put everyone's data in their respective corners.
   • GitLab.com info stays in the gitlab_com directory
   • Self-managed data moves into the page in the admin_area directory
2. Look for crosslinks that should update. So far, looks good.
3. Try not to wince at user/admin_area/settings/account_and_limit_settings.md because oh dear, it needs some love

Related issues

Related to #347004 (closed) where the need was identified by Marcel.

Author's checklist

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