Revisit Sonar for Terasology

[jdrueckert]

Motivation

We do have sonar tasks in our gradle setup, but as @BenjaminAmos mentioned in a comment on #5194, we neither have any documentation on how to run SonarQube scanner on Terasology engine, nor does it seem to be run in CI.

Proposal

We should decide whether or not we want to keep sonar based on the benefits it brings.
If we decide to keep it, we should properly set up the scanner in CI and document how to run it manually.
For the latter case, we can have a look at our website repo where it is integrated as a GitHub app.

Alternatives

If we decide to not keep it, we should remove the related gradle tasks and dependencies from this repository.