Describe theory of embedded mapping

As pointed out [here](https://github.com/Ferrite-FEM/Ferrite.jl/pull/764#discussion_r1350144231), there is some overlap on the mapping of `FaceValues` and embedded `CellValues`.

It would be nice to add some description of how this is calculated, to make it easier to understand the code.
One suggestion would be to add a section in the description [here](https://ferrite-fem.github.io/Ferrite.jl/previews/PR798/topics/FEValues/) after #764 is merged.