Reorder and rename sections in variables doc

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/107808



Merged-by: Kushal Pandya <kushalspandya@gmail.com>
Approved-by: Diana Logan <dlogan@gitlab.com>
Approved-by: Kushal Pandya <kushalspandya@gmail.com>
Reviewed-by: Diana Logan <dlogan@gitlab.com>
Co-authored-by: Marcel Amirault <mamirault@gitlab.com>