Write memo describing coordinate systems and transformations

[plaplant]

When reviewing #840, it occurred to me that we don't have a canonical reference for what we mean when we say ECEF vs ENU vs rotECEF (and that XYZ == ECEF), etc. There is this reference memo in the docs folder, but I don't believe that has the level of specificity required to understand what is meant by the different systems (and doesn't include all systems we support). It would also be good to explain why we use the rotation matrices that we do to convert between the different systems.