Doxygen: Minor layout fixes

- Rename GITLAB->GitLab in header
- Remove the DOI link from README.md as Doxygen currently does not
  handle SVG images correctly. Instead add the DOI link as project badge
  in GitLab directly.
  (https://github.com/doxygen/doxygen/issues/6039, fixed in 1.8.16)
- Add badge for Doxygen documentation. Created and copied from
  https://img.shields.io/badge/Doxygen-master-informational