Add docs coverage and warning checks to CI and resolve failures for both

[willsimmons1465]

Description

Adding CI checks for documentation coverage (making sure that every publicly visible method and class - those whose names don't begin with an underscore - is included in the docs) and enforcing sphinx nitpicky to report any formatting failures and broken cross-references.

This PR also resolves existing gaps in coverage and broken formatting. Some of the newly documented content required for docs coverage may have been intended to be internal components required only as part of the conversion or infrastructure code and is not intended for end users; I will need some help spotting any such instances so we can underscore them out to clarify this intention.

Related issues

This works towards Quantinuum/tket#1857, following on from Quantinuum/tket#1910 for the core package and related PRs for other extensions.

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.

[PabloAndresCQ]

Thanks Will.

Some of the newly documented content required for docs coverage may have been intended to be internal components required only as part of the conversion or infrastructure code and is not intended for end users

Yes, this is the case. I will look through it and add underscores or docstrings where relevant.