docs: add doc for casbin k8s-gatekeeper

[ComradeProgrammer]

Fix: https://github.com/casbin/k8s-gatekeeper/issues/18

[casbin-bot]

@tangyang9464 @JalinWang @imp2002 please review

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[hsluoyz]

@Selflocking plz review

[hsluoyz]

@Selflocking

[hsluoyz]

@ComradeProgrammer