Add workflow to validate with sushi and ig publisher

[vadi2]

Added validation to PRs and any commits to the master branch to ensure that only content that builds an IG successfully makes it in. This will help ensure that this IG doesn't get any broken builds.

Any QA errors or warnings will still be allowed though, and manual approval is necessary for it to run from forks on the first time.

Example of it working in fhircast-docs:

Let me know if there are any questions about this improvement! 🙂

[caspears]

I appreciate the submission. Please help me understand what this buys us. Commits to any branches on the HL7 Github automatically triggers a build with the results posted on Zulip, including the logs of the build (which is useful for troubleshooting). I assume the results of this validation would be made available after an attempted check-in in Github (though I am not familiar with this).
My concern is that we would be creating a workflow that is not generally adopted by HL7 and may cause confusion.
Perhaps this is a request that should be made the the FHIR-I Tooling team.