[Documentation] Katib webhook information

[andreyvelich]

/kind feature

We should document how admission Kubernetes webhooks is used in Katib.

Also, add information about private GKE clusters, see comment: #1258 (comment).

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[andreyvelich]

/good-first-issue

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[connorlwilkes]

With respect to private GKE and webhooks this will be really useful across the board for Kubeflow.

[stale]

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]

/lifecycle frozen