Tutorial on how to implement a new finite element

We should at some point provide at least a section where we describe how new elements can be implemented in Ferrite.

Technically we can also provide, to prottotype new elements, some kind of general Ciarlet element, because the construction of basis functions from a function space is a very mechanistic process (see e.g. https://finite-element.github.io/2_finite_elements.html for a reference). Basically we just have to provide the triple "reference geometry", "moments" and the "function space basis" for which we solve some systems of equations (e.g. vandermonde system for nodal basis functions on a polynomial function space), giving us the coefficients for our basis functions.

We can also show the alternative way via Lagrange polynomials for Lagrange interpolations.