Add docs section for symplectic analysis of Gaussian states and operators

[apkille]

Could discuss symplecticform(::SymplecticBasis), isgaussian and issymplectic checks, changebasis function for permuting symplectic bases, sympspectrum for computing symplectic eigenvalues of a Gaussian state, and eventually symplectic decompositions from SymplecticFactorizations.jl once Gabs is dependent on that package. It would be nice to be pedagogical about symplectic linear algebra in this section.

Update: there's now support for decompositions from SymplecticFactorizations.jl in Gabs (see #27, #28).