This documents:
* The existence of the `lhapdf` package (not particularly important)
* The existence of the `lhapdf.pdf_sets` attrset (important)
* The setup hook used in all of the entries in `lhapdf.pdf_sets`
(slightly important)
Part of #341479.
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
