Update docs with IP ranges

This should only be done after Cloud NAT is rolled out in all "production" (gprd, CI) environments, and has been left for a while to demonstrate stability, and also after we've figured out whether or not contiguous ranges are possible (https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7960).

This doesn't have to wait until https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8083 is completed, but we do need a decision on whether we want to advertise DR ranges or let that be a small inconvenience compared to the chaos we'd experience if we did need to fail over to DR.

Update the docs to advertise IP ranges for customer whitelisting. Do we need to disambiguate gprd (mirroring, webhooks, probably more) from CI?

It is worth exposing the IPs in a machine-parseable way (i.e. don't require systems to scrape HTML). In the interest of simplicity, a static newline-separated plain text list of IPs in our docs might be ideal.

RFC @hphilipps @ansdval

Docs ref: https://docs.gitlab.com/ee/user/gitlab_com/#ip-range