Add documentation for GitLab Pages TLS rate-limits

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118893



Merged-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Kassio Borges <kborges@gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Reviewed-by: Kassio Borges <kborges@gitlab.com>
Reviewed-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: Kassio Borges <kborges@gitlab.com>
Co-authored-by: ngala <ngala@gitlab.com>