[KFTO-SDK] Add doc on customising base images for Train API

[varshaprasad96]

Follow up from https://github.com/kubeflow/training-operator/pull/2261 as this is a user facing change.

[google-oss-prow[bot]]

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[varodrig]

Thanks for adding this @varshaprasad96!

@andreyvelich is this PR ready to be approved?

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