Add docs for GitLab Pages source-IP rate-limits

What does this MR do and why?

Adds documentation for source-IP rate-limits in GitLab Pages

We're currently not enforcing these limits by default and they are guarded by a feature flag. In the case of GitLab Pages, the feature flag is implemented as an environment variable.

I'm not 100% sure we should merge this before we remove the FF. But I wanted to have this MR open as documentation for our own SRE team.

Flags will be available in omnibus though, so we may merge the docs, and just remove the caveat about feature flag later.

gitlab-pages#631 (closed)

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