build: fix Coin 3d documentation discovering
build: fix Coin 3d documentation discovering

* coin3d 4.0.0 default location is /usr/share/doc/Coin/html/
* misuses of find_files() if several candidates for filename
* export variable COIN3D_DOC_FOUND that is used by cMake/FreeCAD_Helpers/PrintFinalReport.cmake

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master