Migrate sanitization command from about.gitlab.com

See https://gitlab.com/gitlab-com/support/support-ops/zendesk-macros/-/merge_requests/199 & review comments here for details.

This MR is a prerequisite to gitlab-com/www-gitlab-com!75638 (merged).

Effectively, these MRs move the usage instructions for the gitlabrb_sanitizer to its Readme.