build: Create octave.desktop even when --disable-docs configured (bug #42720). * etc/icons/Makefile.am, etc/icons/octave-logo.ico, etc/icons/octave-logo.png, etc/icons/octave-logo.svg, etc/icons/octave-sombrero.png, etc/icons/octave.desktop.in, etc/icons/octave_branding_samples.svg: Move doc/icons files to /etc/icons directory. * Makefile.am: Add etc/icons to list of SUBDIRS to build. * configure.ac: Change doc/icons to etc/icons. * doc/Makefile.am: Remove icons from list of SUBDIRS to build.