Add documentation for GitLab Pages TLS rate-limits

I just realized that while pushing hard to get https://gitlab.com/gitlab-org/gitlab-pages/-/issues/632 in production, I completely forgot about adding documentation.

• We should add new limits to https://docs.gitlab.com/ee/administration/pages/#rate-limits.
• And document limits set in https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-com/-/blob/96a3bf149c1cd2847f29c3ed666c480e6b8d15b7/releases/gitlab/values/values.yaml.gotmpl#L482 in https://docs.gitlab.com/ee/user/gitlab_com/#gitlabcom-specific-rate-limits

It probably would be easier to do after gitlab-pages#706 (comment 902872112) since there will be no feature flags involved.