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

[varshaprasad96]

Follow up from kubeflow/training-operator#2261 as this is a user facing change.

[google-oss-prow]

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[google-oss-prow]

Hi @varshaprasad96. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[andreyvelich]

Thanks for adding this @varshaprasad96!

[Arhell]

/ok-to-test