[CDEPS] Update DATM documentation

### What sort(s) of documentation issue is this?
- [ ] Something is missing.
- [x] Something is (or might be) incorrect or outdated.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)
- [ ] [User's Guide](https://escomp.github.io/CTSM/users_guide/index.html) (using the model and related tools)
- [x] Somewhere else (e.g., README file, tool help text, or code comment): _Please specify_
- [ ] I don't know

### Describe the issue
It's actually the [CDEPS documentation about DATM](https://escomp.github.io/CDEPS/versions/master/html/datm.html), which we (will) link to (see https://github.com/ESCOMP/CTSM/pull/3223#discussion_r2126756505), that needs updating. It's missing at least some options—e.g., CRU-JRA— and still contains a reference to Cheyenne. We in CTSM will need to handle this.

See also the CDEPS issue: ESCOMP/CDEPS#341.

Once the CDEPS docs are updated, it would be best if we could get a frozen version of the CDEPS docs for us to link to (see #3192), rather than linking to their "master" version. That way, our clm6.0 docs will always link to the relevant CDEPS docs.