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 added 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
cc @emoyse