Add description of iMOD5 file formats to documentation

[JoerivanEngelen]

We currently lack documentation on what these iMOD5 specific formats are. To know these files are, iMOD5 experience is required. Already to know that this is described somewhere in the iMOD5 manual. At minimum, some information is required so that users can figure for themselves if the IDF/IPF/GEN/PRJ functionality is relevant for them or not.

Not extremely necessary, but nice to have and relatively little effort: iMOD5 documentation can be copied, since these file formats are stable now.