Add docs (GKE part 8)

[richardchen331]

/kind documentation

What this PR does / why we need it:
This change adds relevant docs for the GKE feature in CAPG.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Relates #512

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add documentation for the GKE feature

[k8s-ci-robot]

Hi @richardchen331. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[cpanato]

/retest all

[k8s-ci-robot]

@cpanato: The /retest command does not accept any targets.
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Use /test all to run the following jobs that were automatically triggered:

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In response to this:

/retest all

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[cpanato]

/retest

[cpanato]

thank you!
/approve
/assign @richardcase

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, richardchen331

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[richardcase]

Thanks @richardchen331

/lgtm