Documentation fullsimlight
This MR adds some documentation related to FullSimLight and its tools. It mainly adds a FullSimLight tools section (https://geomodel.web.cern.ch/geomodel/home/fullsimlight/fullsimlight/) that contains subsections explaining the use of the different tools (fullsimlight, gmclash, gmgeantino and gmtogdml). The How to build FullSimLight section is also moved under the For Developers section.