Display doc info pages using documentation browser when in GUI mode (Bug #39451) * scripts/help/doc.m: call __octave_link_show_doc if in gui mode. * libinterp/corefcn/octave-link.h, libinterp/corefcn/octave-link.cc (octave_link::show_doc): New function. (octave_link::do_show_doc): New virtual function. (__octave_link_show_doc__): New function. * libgui/src/octave-qt-link.h (octave_qt_link::show_doc_signal): New signal. (octave_qt_link::do_show_doc): New function. * libgui/src/documentation-dock-widget.cc, libgui/src/documentation-dock-widget.h (documentation_dock_widget::showDoc): New function. * libgui/src/main-window.cc, libgui/src/main-window.h (main_window::construct_octave_qt_link): connect handle_show_doc. (main_window::handle_show_doc): New function. (main_window::show_doc_signal): New signal. * libgui/src/qtinfo/parser.cc, libgui/src/qtinfo/parser.h (parser::find_ref): New function. * libgui/src/qtinfo/webinfo.cc, libgui/src/qtinfo/webinfo.h (webinfo::load_ref): New function.