Add deprecation notice to `mc` model documentation

[jessica-mitchell]

Fixes #2461

Adding deprecation notice to the 'mc' models since the cm_default model is an improved implementation of compartmental neurons, as discussed in issue.

Models should be removed in an upcoming release. Info about NEAT, once that has progressed could also be added.

See Read the Docs output here: https://nest-simulator--3240.org.readthedocs.build/en/3240/models/iaf_cond_alpha_mc.html