element -> cell in transformation functions

To be consistent with the notation in Ferrite,
- `facet_to_element_transformation` -> `facet_to_cell_transformation`
- `element_to_facet_transformation` -> `cell_to_facet_transformation`

(edited to as I mistakenly used cell on both from and to...)

https://github.com/Ferrite-FEM/Ferrite.jl/blob/b8f986a38f0a69d0745af7f4c271453202c8f153/src/FEValues/face_integrals.jl#L1-L15

Since this is only in devdocs, I guess we can change it as a minor version change?

For the documentation, we are also mixing (discussed somewhere else before, but to keep the discussion together, pointing out here too) between cell, element, and (reference) shape. From our convention in #914, I think we ideally should have `ReferenceCell` and not `ReferenceShape`, but since `AbstractReferenceShape` already exists, I guess it makes sense to keep shape for the reference (cells/shapes)? However, I think removing the `element` from https://ferrite-fem.github.io/Ferrite.jl/dev/devdocs/elements/ would make sense.

Thoughts @termi-official, @lijas, @fredrikekre, and more?