Add crosslinks between only/except and rules

What does this MR do?

As per the linked issue, we expect to deprecate the only and except keywords soon. As a first step towards this, we should try to steer users towards using rules instead. This adds crosslinks to all the only/except subkeys to their rules equivalents, explaining that rules is now preferred.

This is already explained at the top of https://docs.gitlab.com/ee/ci/yaml/index.html#only--except, but we want it in the subkeys as well. It'll also help people using those subkeys who want to migrate to rules by leading them to the correct section.

Related issues

Related to #357643 (closed)

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