doxygen navigation bar

[mwiesenberger]

there seems to be an error in the display of the header navigation bar of the doxygen documentation

[mwiesenberger]

This error happened due to an update of the doxygen version. Apparently the tablist class in the css files of doxygen that we use to style the header was removed.
possible solution: copy the tablist class from an older version in a custom css file and use that when creating the documentation.
In the long term we should really look out if there is a way to better integrate the doxygen output into our website

[mwiesenberger]

Added custom css file and changed header in feltor/doc, then updated the documentation on the homepage -> works