Move details about variable usage into keyword reference
Problem to solve
We have an old table at https://docs.gitlab.com/ee/ci/variables/where_variables_can_be_used.html#gitlab-ciyml-file that lists which keywords can use variables. We should move all that into the keyword details themselves, in their Possible inputs
sections, and drop this table completely.
Pointed out in #34272 (comment 716011456)
Further details
Proposal
Who can address the issue
Other links/references
Edited by Marcel Amirault