Markdown checks are not executed for PRs making changes in api directories of the operators

[bacherfl]

Currently, markdown checks only seem to be executed when something in the docs has changed, but not when a struct for defining one of the CRDs has been updated in operator/apis. or metrics-operator/api.

This can cause issues if e.g. a PR only changes something in the go structs, which will cause a subsequent PR making changes to the docs to fail this test, as at this point the CRD documentation will be out of sync.