Add .readthedocs.yaml configuration file

[marcfrederick]

Read the Docs has started requiring a .readthedocs.yaml configuration file to be present in the repository in order to build the documentation.

https://blog.readthedocs.com/migrate-configuration-v2/

According to the timeline we have until 2023-09-25 to migrate to the new configuration file, but we've already had some builds fail during their 48-hour brownout on 2023-09-04.

[AdamWill]

I tend to hate hardcoded versions which inevitably go hilariously stale over the years...but at a quick look it seems like there aren't any 'latest' or 'stable' redirects we can use for these, so we're just stuck. :(

[marcfrederick]

Yep, we're pretty much stuck with this -- the setup is more or less copy-paste. The migration guide makes it sound like the requirements.txt is only required for reproducible builds, but without it I've had build failures, so I guess it's required.

[waldyrious]

FYI, I recently got an email about our ReadTheDocs integration — it seems to be for a separate issue than what's handled in this PR (I've forwarded it to you both so you can check), but I thought I'd take the opportunity to mention publicly that besides access to the mwclient GitHub organization and the mwclient PyPI project, it's also necessary to ensure maintainers have access to the mwclient project on ReadTheDocs.

Hopefully we will later create a proper documentation for project maintenance, mentioning these account access steps that need to be performed when adding new maintainers, the preferred PR merge strategy, etc.; but in the meantime, hopefully this comment will offer something searchable.