Docs docs: Add tip: Math macros

### 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- [ ] Something is (or might be) incorrect or outdated.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
At least in MyST Markdown, you can define a "math macro" at the top of a document like so:
```md

$$\newcommand{\gddmat}{GDD_\textrm{mat}}$$
```
This will allow you to simply do `$\gddmat$` to get $GDD_\textrm{mat}$.