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

README.md

-[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.2641997.svg)](https://doi.org/10.5281/zenodo.2641997)
-
-
-
 ATLAS Offline software
 ========================
 

docs/Doxygen-master-informational.svg

+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="108" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="a"><rect width="108" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#a)"><path fill="#555" d="M0 0h59v20H0z"/><path fill="#007ec6" d="M59 0h49v20H59z"/><path fill="url(#b)" d="M0 0h108v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="110"> <text x="305" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="490">Doxygen</text><text x="305" y="140" transform="scale(.1)" textLength="490">Doxygen</text><text x="825" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="390">master</text><text x="825" y="140" transform="scale(.1)" textLength="390">master</text></g> </svg>
\ No newline at end of file

docs/DoxygenLayout.xml

@@ -19,7 +19,7 @@
       <tab type="filelist" visible="yes" title="" intro=""/>
       <tab type="globals" visible="yes" title="" intro=""/>
     </tab>
-    <tab type="user" visible="yes" url="https://gitlab.cern.ch/atlas/athena" title="GITLAB"/>
+    <tab type="user" visible="yes" url="https://gitlab.cern.ch/atlas/athena" title="GitLab"/>
     <tab type="user" visible="yes" url="http://acode-browser1.usatlas.bnl.gov/lxr/source" title="LXR"/>
   </navindex>