Maintenance: Github workflow for automatic build of app and build checks

[ralf401]

For external contributions via PRs, there should be a Github workflow which builds the app automatically (like for the RTD branches, where it is already in place). This check should also perform build checks as in the internal pipeline to immediately show if the changes are ok. This makes it easier to review changes and for externals to see if they've done it in a way that it can be built.

Additionally, consider posting build artifacts as a comment to the PR automatically. This was already implemented for old repos in zammad/zammad-documentation@7a464a9.