Docs docs: Link to new OnDemand docs

### 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- [ ] Something is (or might be) incorrect or outdated.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
The NCAR Open OnDemand service has now officially launched, and there is [documentation](https://ncar-hpc-docs.readthedocs.io/en/latest/compute-systems/ood/) available. This should be linked in the ["Building and previewing the documentation on Casper"](https://escomp.github.io/CTSM/users_guide/working-with-documentation/bld-prev-docs-casper.html) section of the User's Guide.