Update GP doc notation

[drezap]

Summary:

I'm going to change a lot of the notation for the GP docs to be the similar to BDA3 and GPML. These are international references for Bayesian inference and applied GP's, respectively, and I'm going to unify the notation with these books. It shouldn't all the sudden change when people get to our docs.

Current Version:

v2.18.0

[jgabry]

I think that's a good idea, thanks!