Feature: deployment setup for scorecard-site

[azeemshaikh38]

I plan to make some changes to how scorecard-site gets deployed. This is what I want to do:

1. Have 2 AppEngine configurations: prod and staging.
2. main branch in this repo will update the staging site while production-site branch will update prod. This will be done through CloudBuild triggers.
3. Branch protection setting for both main and production-site will require code review and approvals. Any changes which need to be pushed to production site, will require a PR from main to production-site.

We can carry over this process when we are ready to port over to Netlify.

@mdunbavan @ossf/scorecard-maintainers FYI.

[mdunbavan]

@azeemshaikh38 this all seems legit and a good way going forward for now.