p-multigrid example

[termi-official]

I think it would be nice to develop an example where we show how to do p-multigrid in Ferrite. This technique is nice for matrix-free stuff (xref https://github.com/Ferrite-FEM/Ferrite.jl/pull/766), because the hierarchy is given by the function space seqence (e.g. Lagrange order 4,2,1) where we can just apply A implicitly. However, we can start with a direct solve on coarse level and think later about how to deal with the matrix-free case.