API reference: detect missing class pages & populate all gaps

[sadielbartholomew]

We have a top-level script check_docs_api_coverage calling docs/source/check_docs_api_coverage.py to detect any udocumented methods and missing .rst stub files for the API Reference of the documentation, however it doesn't detect any classes which are present in the codebase which aren't documented, so whole classes and methods defined on them may be missed out.

This PR adds a check for missing classes to the script (excluding classes which represent our user-facing functions, emerging as docstring-substitution metaclasses) and then has used this to detect missing cases and added those in (turns out ~10 classes were missing).

As part of this, close #957.

Opening as a draft since I still have yet to sort:

• why the Domain class is not accessible/clickable;
• why the methods now listed on new class stub files for the previousy-missing classes are not yet accessible/clickable.