Add readthedocs yaml file

[sjspielman]

Purpose

This PR adds the .readthedocs.yaml file. Linking to RTD sooner rather than later will be good for review (see #58 (review)).

Issue Addressed

Towards #5; I will close this out issue manually after actually linking to RTD.

Any comments, concerns, or questions important for reviewers

After this goes in, I plan to formally import the repo to RTD. During that process, I'll add other folks (Jackie, Ally, Josh, David) besides me as maintainers. Are there any other specifics I'll need to know there?

[jashapiro]

LGTM

(...but you might want to make yourself a local precommit file so you can add the ending newline hook)

[sjspielman]

but you might want to make yourself a local precommit file so you can add the ending newline hook)

💯 thanks for the reminder