Changes:

• upgraded warning in cmake
• upgraded Doxyfile, header, footer and stylesheet using doxygen + manual changes

  doxygen -w html header.html footer.html stylesheet.css
  doxygen -u -s Doxyfile.in
  

  • enabled clipboard copy for code snippets
  • used complete new CSS stylesheet from doxygen with slight changes: use old fonts, fixed content width, extra margins and padding in some places, re-formatted and removed empty styles

Note: new stylesheet enables dark mode support

Note: this PR shouldn't be ported to 5.x, similar changes should be made manually

Note: also fixed documentation warning in highgui.hpp (introduced here #26524)