docs: tweak 404 page
docs: tweak 404 page

the docs are responsive, so on small screens the version is hidden in the menu. 
replaced <pre> with <code> as this does not cause new lines.
MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3