Better docs builds

[KrystalDelusion]

What are the reasons/motivation for this change?

• The current setup for rtds_action doesn't work locally.
• There are also cases where ReadtheDocs will build before the docs artifact is ready and upload an incomplete version of the documentation.

Explain how this is achieved.

• Gate rtds_action behind checks for both READTHEDOCS and GITHUB_TOKEN environment variables. This also has some changes to (better) support building on mac and other small formatting fixes.
• Update .readthedocs.yaml to fail if sphinx raises any warnings, notably because it's missing files. This also involves fixing the couple of warnings that were still popping up. I also ended up splitting the prepare-docs job from the test-verific in order to better control when the job would run. There is an extra change so that todos appear in preview builds but not in release builds.

If applicable, please suggest to reviewers how they can test the change.
Run make docs locally. Check docs-previewtest tag (https://yosyshq.readthedocs.io/projects/yosys/en/docs-previewtest/).

[KrystalDelusion]

As a note, I was running export "LC_ALL=C" make docs PYTHON=python. The LC_ALL was to resolve an error about an unsupported locale when using sphinx, and the PYTHON=python was because I was getting an error when trying to python3/pip3 install about it being externally managed and suggesting using a venv instead, whereas python/pip didn't... Not sure if either of those are because mac, or specific to my partner's machine/setup.