Add documention about the ATLAS Extensions

documentation/docs/fullsimlight/atlas-extensions/index.md

+# ATLAS Extensions
+
+FullSimLight is an experiment agnostic detector simulation tool that has been implemented and is mantained by the ATLAS collaboration. All the ATLAS specific customizations are available at the [ATLAS Extensions](https://gitlab.cern.ch/atlas/geomodelatlas/ATLASExtensions) repository that is linked to the GeoModel repository as a git submodule. In order to have the corresponding code cloned inside the GeoModel repository the user would need to clone recursively the main repository. So either:
+
+If you still didn't clone the repository you can clone it recursively:
+
+```c++
+git clone --recursive https://gitlab.cern.ch/GeoModelDev/GeoModel.git
+```
+Alternatively, if you already cloned the GeoModel repository, it will be sufficient to type the following commands from the main repository folder:
+
+```c++
+git submodule init
+git submodule update
+```
+
+A flag is available to build the ATLAS extentions (by default OFF) and it should be turned ON in order to build all the extensions:
+
+```c++
+cmake ../ -DGEOMODEL_BUILD_ATLASEXTENSIONS=ON
+```
+
+A configuration file {{atlas-conf.json}} with all the ATLAS specific customizations is provided with the ATLASExtensions and will be installed under {{<install>/share/FullSimLight/ATLAS}}. In order to run a Geant4 ATLAS with your favourite geometry tag you only have to run the following:
+```c++
+./fullSimLight  -g geometry.db -c atlas-conf.json
+```
+
+## ATLAS Geometry File
+Some ATLAS geometry SQLite files are available at the following [link](https://geomodel.web.cern.ch/atlas-geometry-data/), only worrking under CERN SSO.
+They describe the whole geometry tree of different tags used during the different LHC runs. You can select your favourite one and use it with fullsimlight!
+
+## ATLAS Magnetic Field Map and plugin
+<!---->
+This folder contains the ATLAS magnetic field map implementation. It uses a magnetic field map data file that is available at the following [link](https://geomodel.web.cern.ch/atlas-magnetic-field/) and can be downloaded with wget:
+```c++
+wget https://geomodel.web.cern.ch/atlas-magnetic-field/bmagatlas_09_fullAsym20400.data
+```
+## LAr Custom Solid Extension
+<!---->
+This folder contains the LArCustomSolid Extension that describes the ATLAS LAr EMEC custom shape. The LAr EMEC custom shape extension is build starting from a copy of the corresponding code in the Athena repository.
+To keep the code of the LArCustomSolid Extention synchronized with the latest available version in the Athena repository, just run the autonomous-lar.sh script.

documentation/mkdocs.yml

@@ -39,6 +39,7 @@ nav:
         - 'fsl': 'fullsimlight/fsl/index.md'
         - 'Plugin Examples': 'fullsimlight/plugins/index.md'
         - 'Plugin Support': 'fullsimlight/plugin-support/index.md'
+        - 'ATLAS Extensions': 'fullsimlight/atlas-extensions/index.md'
     - FAQ: 'faq.md'
     - For Developers:
         - 'Build the Libraries and Tools': 'dev/index.md'