Document using labels to enhance kanister logs context

[hairyhum]

Currently kanister controller includes actionset labels with prefix kanister.io/ into controller logs when logging events for actionsets. https://github.com/kanisterio/kanister/blob/master/pkg/controller/controller.go#L391

This is not documented anywhere at the moment. Would be nice addition to something like https://docs.kanister.io/tasks.html

NOTE As we move towards the new docs format, docs changes should be repeated in docs and docs_new directories.

[viveksinghggits]

Looks good to me, while documenting we can also mention some use cases.