Add the mdformat packages to the pre-commit hook needed for that. Format
the existing files.

After migrating to GitLab.

Also, set `--fail-if-warnings` to fail on warnings.

Also an example file for testing purposes and some Hebrew fonts.

- Use newer runas script, using gosu instead of sudo.
- Added test steps to the build using the example presentation (in fact
using Docker Hub as a sort of CI).
- Use newer Golang image.
- Small corrections to README.
- More streamlined Makefile.
- Added groff to image.
- Added XeTex to image.