Add Pages access control user docs

What does this MR do?

EE MR of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23145. It should be merged in EE with the CE>EE merge https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8542, but not sure how much this will take, so let's best create an EE MR and have the docs updated.

Related issues

https://gitlab.com/gitlab-org/gitlab-ce/issues/54143

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards