chore: add .readthedocs.yaml

[alcarney]

Description (e.g. "Related to ...", etc.)

Unsurprisingly, #359 broke the docs build - especially when you consider it was still running on Python 2.7!

This PR adds a .readthedocs.yaml file, which is now the recommended way to configure builds on the platform.

The docs/requirements.txt file was generated using a poetry export --with docs command, so we'll have to remember to periodically update it - unless we find a way to automate its generation.

Code review checklist (for code reviewer to complete)

• [ ] Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• [ ] Title summarizes what is changing
• [ ] Commit messages are meaningful (see this for details)
• [ ] Tests have been included and/or updated, as appropriate
• [ ] Docstrings have been included and/or updated, as appropriate
• [ ] Standalone docs have been updated accordingly

[tombh]

Ohh, that makes sense, it runs on a webhook, so it's not so easy to remember to keep up to date. Should be easy to automate updating the requirements.txt file if we keep forgetting to update it.

I think this branch went out of date with the merge of #360.

[tombh]

Pyodide test passed on a re-run. It was a new flakey error, I should have noted it down, in case we see it again.

Edit: Noted the error in https://github.com/openlawlibrary/pygls/issues/367