doc/packages/geant4: add documentation

This documents:
* The `geant4.data` attrset introduced in #39514
* The setup hook for geant4
* The setup hook for the geant4.data packages

I also added a brief description of what Geant4 is.

Part of #341479

doc/packages/geant4.section.md

@@ -0,0 +1,15 @@
+# Geant4 {#geant4}
+
+[Geant4](https://www.geant4.org/) is a toolkit for simulating how particles pass through matter. It is available through the `geant4` package.
+
+## Setup hook {#geant4-hook}
+
+The setup hook included in the package applies the environment variables set by the [`geant4.sh` script](https://github.com/Geant4/geant4/blob/master/cmake/Modules/G4ConfigureGNUMakeHelpers.cmake#L4-L55), which is typically necessary for compiling `make`-based programs that depend on Geant4.
+
+## Datasets {#geant4-datasets}
+
+All of [the Geant4 datasets provided by CERN](https://geant4.web.cern.ch/support/download) are available through the `geant4.data` attrset.
+
+### Setup hook {#geant4-datasets-hook}
+
+The hook provided by the packages in `geant4.data` will set an appropriate environment variable in the form of `G4[...]DATA`. For example, for the `G4RadioactiveDecay` dataset, the `G4RADIOACTIVEDATA` environment variable is set to the value expected by Geant4.

doc/packages/index.md

@@ -12,6 +12,7 @@ emacs.section.md
 firefox.section.md
 fish.section.md
 fuse.section.md
+geant4.section.md
 ibus.section.md
 kakoune.section.md
 krita.section.md