User's Guide: How to calculate above/belowground biomass

### What sort(s) of documentation issue is this?
- [x] Something is missing.
- [ ] Something is (or might be) incorrect or outdated.
- [ ] Something is confusing.
- [ ] Something is broken.

### What part(s) of the documentation does this concern?
- [ ] [Technical Note](https://escomp.github.io/CTSM/tech_note/index.html) (science and design of the model)
- [x] [User's Guide](https://escomp.github.io/CTSM/users_guide/index.html) (using the model and related tools)
- [ ] Somewhere else (e.g., README file, tool help text, or code comment): _Please specify_
- [ ] I don't know

### Describe the issue
A common question on the DiscussCESM forums is how to calculate aboveground or belowground C or N, since we don't have output variables corresponding to those. We should have standard instructions in the User's Guide.