Add Docs about combining kernels for Gaussian Processes

[drezap]

Summary:

I'll include valid kernel operations, proper implementation, and an illustrative time series example with statistical graphics. Notation will be unambiguous.

Users should be able to copy and paste code, and include in their own models, for example, if they want to use a Gaussian Process prior on a parameter in a larger model.

For statistical graphic colors, I'll try to keep to Stan's red theme, that can be found in Betancourt's case studies. Colors will be accommodating for those with color vision deficiency.

Current Version:

v2.18.0