Add theory on FE-spaces in docs

As pointed out by @termi-official in https://github.com/Ferrite-FEM/Ferrite.jl/pull/1140#discussion_r1958685996, it could be nice to

> include some kind of reference on how to construct the interpolations in first place (e.g. https://finite-element.github.io/L2_fespaces.html or something else).

Probably in the devdocs section for now if we want to refer to internal API for interpolations, but once #626 is resolved and we have a public "new interpolation API", we could move this out.