Re-organize rate limits documentation

Problem to solve

The rate limits documentation could use some refactoring and re-organization. Currently, we have many items scattered all over the docs. We should at least group the pages under the same nav to be easier to navigate (mvc: gitlab-docs!4998 (merged)).

Further details

While working on gitlab-docs!4992 (merged), we realized one nav item was misplaced.

This led to opening an MR to place all rate limits under the same nav gitlab-docs!4998 (merged). However, there are still some things to clean up.

Proposal

• Merge https://docs.gitlab.com/ee/administration/settings/rate_limits.html and https://docs.gitlab.com/ee/security/rate_limits.html as the former is only a list of pages. The doc under security describes more in depth about the rate limits. We then can redirect one page to the other (discussion).

• We have two pages about Git abuse rate limits, which I believe we should eventually merge:

  • Group-wide: https://docs.gitlab.com/user/group/reporting/git_abuse_rate_limit
  • Instance-wide: https://docs.gitlab.com/ee/administration/reporting/git_abuse_rate_limit.html

  The group one has a section that's almost identical to the instance one.

  There's also almost no information on how GitLab determines how a user is banned. There's a linked closed epic which is currently confidential.

• Make all H1 headings consistent. Currently, we have a mix of Rate limits on X and X rate limits. We should be consistent and pick one.