docs: Add vouch-proxy OAuth example

[domdomegg]

What this PR does / why we need it:

Adds an example of OAuth authentication with Vouch Proxy, which supports a few providers not supported by oauth2_proxy.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

Fixes #10867

How Has This Been Tested?

Validated that this works on a local cluster correctly.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

/area docs
/kind documentation

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[k8s-ci-robot]

Hi @domdomegg. Thanks for your PR.

I'm waiting for a kubernetes 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[longwuyuan]

/ok-to-test

[longwuyuan]

/triage accepted
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: domdomegg, longwuyuan

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Needs approval from an approver in each of these files:

• docs/OWNERS [longwuyuan]

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