Document `Dihedral_angle_cosine` from Tetrahedral_remeshing

[janetournois]

Dihedral_angle_cosine might be useful for other tet-related packages. Do you plan to make it more official one way or another?

Originally posted by @lrineau in #7830 (comment)

[lrineau]

Maybe not document it, but move it in a dedicated header, and maybe document it internally for other developers

[janetournois]

Algebraic Foundations is probably the right package for that.
Dihedral_angle_cosine is just a squared fraction, with the sign of the non-squared version.