remove kale docs (project is abandoned)

[thesuperzapper]

The kale project has not been updated since Oct 20, 2021, therefore I think we can treat it as abandoned and remove it from the Kubeflow website.

Also, there are many issues on the kubeflow-kale/kale repo discussing that the project is no longer maintained:

• Is Kale dying? kubeflow-kale/kale#436
• Readme Does Not Disclose Kale's Dependency on Arrikto EKF kubeflow-kale/kale#395

[thesuperzapper]

@chensun @zijianjoy @james-jwu @jbottum, I would appreciate a quick LGTM / approval on this one so we can stop promoting an abandoned project.

[jbottum]

@StefanoFioravanzo @akgraner @kimwnasptd @cvenets please confirm this or provide an update on Kale, thanks

[zijianjoy]

/lgtm
/approve

It makes sense to avoid promoting an abandon project. If this is done by mistake, please feel free to create a new PR for bringing it back.

[google-oss-prow]

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