Improve development docs regarding managing RubyGems.org footprint

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144768



Merged-by: Ahmed Hemdan <ahemdan@gitlab.com>
Approved-by: Philippe Lafoucrière <30915-plafoucriere@users.noreply.gitlab.com>
Reviewed-by: Philippe Lafoucrière <30915-plafoucriere@users.noreply.gitlab.com>
Reviewed-by: Rémy Coutable <remy@rymai.me>
Reviewed-by: Ahmed Hemdan <ahemdan@gitlab.com>
Co-authored-by: Philippe Lafoucrière <30915-plafoucriere@users.noreply.gitlab.com>
Co-authored-by: Rémy Coutable <remy@rymai.me>
Co-authored-by: Jayson Salazar Rodriguez <mail@jdsalaro.com>