KEP-2170: Update documentation for V2 APIs

[andreyvelich]

Related: #2170

We should update our docs for Training Operator V2 APIs, and add the information for user migration from V1.

/area docs

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[andreyvelich]

/remove-lifecycle stale
/good-first-issue

[google-oss-prow]

@andreyvelich:
This request has been marked as suitable for new contributors.

Please ensure the request meets the requirements listed here.

If this request no longer meets these requirements, the label can be removed
by commenting with the /remove-good-first-issue command.

In response to this:

/remove-lifecycle stale
/good-first-issue

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[palSagnik]

/assign

I would like to tackle this issue, and as this is my first time contributing here can I know what exactly is expected for this issue?