[doc] Large number of uncategorized files in Fluidmatrixinteractions group

Feature request

I propose to add more subgroups to the Fluidmatrixinteractions group to make it easier to see what kind of constitutive relations are contained in this group.

What does this feature / why does DuMux need it:

Currently, there is a large number of files directly in the Fluidmatrixinteractions group and it is hard to get an overview of what is available. Introducing the subgroups will also bring the organization of the documentation closer to the internal folder structure of Dumux, where the files are already grouped.