Documentation urls for ml.validate, ml.validate_detector

[russcam]

In updating the REST API specs in #54252, I've noticed that there are no documentation URLs for

• ml.validate
• ml.validate_detector

It would be good to have documentation URLs in the specs, as these are used by clients when generating components. I wasn't sure which reference pages to link them to.

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[russcam]

This was addressed in #55749. Closing.