Verification github action for PR

With the new changes in the `hakyll` branch, we have an infrastructure that can build and then deploy to the main branch.

Next we need the same kind of CI for PRs into that branch that does the build, and hopefully later some verification steps, but no deploy.

Start by reviewing the existing build-and-deploy infrastructure on the hakyll branch and how it is triggered. Add equivalent PR verification that builds without deploying, with successful PR builds and no deployment as the completion criteria.

Scritto dal modello di indicizzazione a partire dal testo della issue.