Add readthedocs.yaml config file

[Zh-xy]

Quoting Fridolin's message on Slack, "We have been notified by ReadTheDocs that they are going to deprecate documentation builds that don't use the latest version (v2) of their configuration file. message-ix-buildings is one of two of our projects that doesn't already do that. They have posted a blog article detailing why and when this happens and how such a config file can be created. Please let me know if you have any questions about dealing with this.
Config files in the required format already exist for message_ix, ixmp, message-ix-models etc (though I just noticed that all of them are called .readthedocs.yml instead of .readthedocs.yaml and we should probably rename them) if you want some inspiration. This is not urgent, as the full deprecation will only happen in September, but partial deprecations will happen before that (the first one on July 24), so best not to postpone this too much."

[glatterf42]

Just bumping this because I've got an email reminder about adding a config file for RTD for this repo again.

[amastrucci]

@Zh-xy could you take the lead on this?

[Zh-xy]

Hi all, I've created a .readthedocs.yaml file to be added to the main branch - your reviews are needed to commit to main. If you have any comments or want to add more configuration info, feel free to let me know. We can edit it anytime. Cheers!