Documentation update for switching GCHP default resolution to c90 - #557

Tagging @sdeastham to make sure descriptions + citation in supplemental guide are sufficient.

@mcdon1718-h this looks good to me but I'll let @lizziel and @sdeastham chime in.

I would avoid referring to cubed-sphere resolutions as being equivalent to an LL resolution. I'd instead say something like "C90 (for example) has a resolution of approximately 110 km globally, similar to the average resolution of a 1x1 degree lat-lon grid".

I would also suggest being a little careful in general. I'm certainly in favor of the standard resolution being C90, but there's a strange inconsistency in saying "performance is degraded at C24 and C48" if we don't acknowledge that this also applies to GC-Classic. Specifically, the wording "As there are known issues with representation of some transport processes at coarser model resolutions" seems to imply that the issue is FV3 at C24/48. I would probably instead say "Users should choose their resolution with care, and higher-resolution simulations inevitably provide a more faithful representation of transport". It's a (very difficult!) research question to say what is a "sufficient" resolution; for some problems C30 might be fine, and for others you might really need C720.