Add/improve docs for new meshes (wedge, cut-cell)

[jlchan]

• [x] New wedge/prism meshes
• [x] hybrid, cut-cell, and non-conforming meshes in StartUpDG.jl.
• [x] New approximation type: TensorProductWedge

[jlchan]

Wedge meshes: make sure to document that mapM is a matrix of UnitRanges now instead of a matrix of Int64.

Edit: this comment is now outdated