Write documentation on changes in model output times for CESM3

[billsacks]

As @phillips-ad points out, we should be sure to have extensive and very visible documentation on the changes in model output times in CESM3 (https://github.com/orgs/ESCOMP/projects/2/views/11). We should explain the changes, give rationale, and possibly give some guidance for how people might need to change their post-processing scripts to account for these changes.

This should be called out visibly in the summary of changes for CESM3 so that users are likely to see this. (Some of these changes could cause existing post-processing scripts to give wrong results if users don't know they need to change something – especially the change in time value for time-averaged fields.)