Add CI pipeline to check for build errors

[mmerklinger]

This PR introduces a CI pipeline with two checks.

1. Checks a full build of the documentation for warnings and errors during build.
2. Checks the syntax with the rstcheck tool.

The first check should from now on be mandatory to pass. The second gives and indicator if a merge introduces other problems and helps with future improvements. As it currently fails it has continue-on-error set.