Example of using DelaunayTriangulation.jl for mesh generation

[DanielVandH]

Closes https://github.com/Ferrite-FEM/Ferrite.jl/issues/975

Wasn't too sure where to put the example, so I just put in the howto section.

I explain some things that are also explained in more detail in DelaunayTriangulation.jl's docs since I wanted to try to make this example somewhat self-contained, but I give links to my docs for more information. The mesh refinement here is pretty simple, but I note in the explanation that it could be easily adapted for things like graded meshes or adaptive refinement.