Docs: `Domain` & `DomainList` classes ref. not accessible

[sadielbartholomew]

The Domain and DomainList classes aren't clickable in the docs at present (at least, via https://ncas-cms.github.io/cf-python/function/cf.read.html and some other locations), where:

• DomainList isn't listed under ../docs/source/class so needs a stub file added there and that might be the fix;
• Domain does have a stub file, so the reason it isn't clickable is not as simple as that and needs investigation.

As noticed in #935 (review).