website: Added workflow to run GitHub Actions with /ok-to-test

[hbelmiro]

Checklist:

• You have signed off your commits
• Ensure you follow best practices from our guide. Contributing.
• You have included screenshots when changing the website style or adding a new page.

Description of your changes:
Currently, the /ok-to-test command doesn't trigger GitHub Actions. This PR adds that ability.
The workflow that this PR adds was copied from https://github.com/kubeflow/pipelines/blob/master/.github/workflows/pr-commands.yml.

Issue

Closes: #

Labels

/area website

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[google-oss-prow]

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[hbelmiro]

@andreyvelich can you PTAL?

[andreyvelich]

Do we need to also add this action to other repos (e.g. kubeflow/trainer) @thesuperzapper ?

[andreyvelich]

I can see that notebooks use slightly different action: https://github.com/kubeflow/notebooks/blob/main/.github/workflows/gh-workflow-approve.yaml