Use PR to submit readme/tag changes to mcrdocs repo

[mthalman]

Currently Image Builder updates readmes and tag YAML files in the mcrdocs repo with a direct commit. There was recently a case where such a commit introduce duplicate tag references that results in a PR validation error. But the commit that made the change bypassed any kind of validation. This resulted in other people getting validation errors for their PRs in that repo.

One way to prevent this from happening is for Image Builder to no longer directly commit changes to the repo and instead submit a PR that undergoes validation. If the validation succeeds, then the PR can be automatically merged. One challenge with this approach is needing to avoid cases where a validation error unrelated to our changes prevents the PR validation from succeeding. We would not want that to fail our build.