spec: describe pairing groups in cryptography section

[redshiftzero]

We should add details about the BLS12-377 pairing groups we're using for Penumbra, e.g. the generator points we're using in G1 and G2, to the spec under the cryptographic protocol section. These choices are coming from upstream Arkworks. See motivation here where @cronokirby discovered another project also using BLS12-377 but with different generator points.