Add documentation for Google Cloud runner platform

What does this MR do and why?

This MR adds documentation for Google Cloud runner platform. This functionality can be seen in action in https://gitlab.com/issue-reproduce/google-cloud-integration/runner-cloud-provisioning/-/runners/new.

EE: true

Part of #428498

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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n/a

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

https://main-ee-146517.docs.gitlab-review.app/ee/ci/runners/provision_runners_google_cloud.html