This is the first PR adding vendor-specific documentation.

- Refactor installation docs to make it simpler with tabs for generic
  instructions, Google cloud, and Openshift. This is a multi-PR change.
  The actual installation doc is created on the Pipelines repo and
  rendered on the website.

- Update the build and push how-to with instructions to authenticate to
  Google Artifact Registry.

- Create a new shortcode to commenting out text in the markdown file.