Improvements for mobile and dark mode

Added

• Improved configuration documentation
• License headers in CSS-files

Changed

• Long words in the header are now force-wrapped to to improve readability on small screens
• The searchbars contrast has been improved in dark mode:
  
• Code fragments do not wrap content any more, instead they scroll horizontally if content is too long. On small screens fragments also do waste less space left and right:
  
  If you want to restore the previous behaviour where lines did wrap if they where too long, set the CSS-variable --fragment-whitespace to initial.
• Member function interfaces do wrap better on small screens.

Fixed

• Code fragments do not force a horizontal scrollbar on the website if the fragment lines are too long. Instead the do have a scrollbar themselves.
• In the Sidebar-only variant on touch-devices the sidebar does not show a big touch-optimized resize bar any more. It is not needed because the sidebar width is fixed.
• The navigation breadcrumps do not shrink to the right of the searchbar if GENERATE_TREEVIEW is disabled:
  
• The Content is not overlapping with the footer in the sidebar-only variant any more.