User's Guide: Generating land use timeseries with subset_data

The [`subset_data` User's Guide section](https://escomp.github.io/CTSM/users_guide/running-single-points/generic-single-point-regional.html#subset-data) doesn't include instructions about how to create landuse files.

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