Create documentation for xgboost operator

[richardsliu]

Should be added here: https://www.kubeflow.org/docs/components/

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[richardsliu]

/assign @merlintang

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[merlintang]

thanks @richardsliu , I am so glad to write this document.

[jtfogarty]

/priority p2

[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.

[sarahmaddox]

We should keep this issue open, as we're currently working on graduating XGBoost to v1.0. See kubeflow/xgboost-operator#44

[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.