Move the private GKE docs to the website.

[jlewi]

Related to kubeflow/website#1705

[kubeflow-bot]

This change is 

[jlewi]

/assign @Bobgy

[jlewi]

Preview:
https://deploy-preview-2190--competent-brattain-de2d6d.netlify.app/docs/gke/private-clusters/

[Bobgy]

/hold
/lgtm
We should merge after the website PR is merged

[jlewi]

Website PR has been merged.

/hold cancel

[Bobgy]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Bobgy, jlewi

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[k8s-ci-robot]

@jlewi: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
kubeflow-gcp-blueprints-presubmit	f35d6e0	link	/test kubeflow-gcp-blueprints-presubmit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[jlewi]

Merging manually since this isn't related to the test failure.