Docs: deadlinks to defelement.com in interpolations.jl

[jfdev001]

In src/interpolations.jl there are dead links for sources of reference finite elements (e.g., src/interpolations.jl#L670)

function reference_shape_value(ip::Lagrange{RefQuadrilateral, 3}, ξ::Vec{2}, i::Int)
    # See https://defelement.com/elements/examples/quadrilateral-Q-3.html
    ...

These links could either (1) be removed (not ideal since obviously the information at the links inspired the function bodies), (2) should be replaced with a new source (using the same website now www.defelement.org or a different source), or (3) replaced with a wayback machine link (e.g., https://web.archive.org/web/20220817132305/https://defelement.com/elements/examples/quadrilateral-Q-3.html).

Option (3) is the easiest, and I can open a PR for this. Option (2) may not be feasible since the new website www.defelement.org does not seem to provide the functionals and basis functions for desired interpolation on an element (see e.g., https://defelement.org/elements/examples/quadrilateral-lagrange-equispaced-3.html). I could also open an issue in the DefElement repo since not listing the functionals and basis functions of shape functions on a particular element type is not ideal.

[KnutAM]

Nice catch!
Using the wayback temporarily and opening a pr/issue at defelement seems like the best option to me.