Split DEM and PIC out of the solids page; document solids.model and dem.lsd

The solids page keeps the types and their properties. New DEM model and PIC model pages (separate panes in the GUI) hold the model inputs: dem.contact_model (lsd), the linear spring-dashpot parameters under the prefix dem.lsd, the shared friction and tangential history inputs, the restitution coefficients for every solids type, and the PIC inputs. solids.dem and solids.pic are gone; solids.model, set in the particle options, applies to every type.

See https://mfix.netl.doe.gov/gitlab/exa/mfix/-/merge_requests/1957