Replace deprecated `only/except` with `rules` from our documentation

Problem

The only/except keyword was deprecated in 13.0 however we still provide examples on how to use it on our documentation, hence encouraging users to continue using this keyword.

Solution

While the keyword is not deprecated, we still need to maintain docs about it, but we can try to start pushing people away from it more strongly. We should:

1. Add more crosslinks from specific only/except references (refs, variables, changes, exists) to the rules reference equivalents. -> !90698 (merged)
2. Add docs about migrating from only/except to rules. Add mirror examples for every only/except example, but using rules instead.
3. Deprecate the keywords (Milestone TBD).