Document the spec.managedBy field and its use for MultiKueue

[mimowo]

What you would like to be added?

This is to document the feature implemented under #2193.

The new feature is only supported for MultiKueue and so we may wait with the documentation until:

1. the feature is released in the next release of training-operator
2. Kueue is compiled against the new operator to leverage the new field

The documentation pages should cross-references each other.

Why is this needed?

To help users of the training-operator to know about this feature, and how to use it for Multi-Cluster Job dispatching.

[mimowo]

/cc @andreyvelich @tenzen-y @mszadkow

[andreyvelich]

/area docs
/remove-label lifecycle/needs-triage