Update docs CI to use conda
This should provide a newer version of sphinx and dependencies which isn't ancient, and allows docs to be built locally much easier as well.
Maintenance will be performed on git.ligo.org, containers.ligo.org, and docs.ligo.org on Tuesday 30 April 2024 starting at approximately 9am PDT. It is expected to take around twenty minutes and there will be several periods of downtime throughout the maintenance. Please address any comments, concerns, or questions to the helpdesk.
This should provide a newer version of sphinx and dependencies which isn't ancient, and allows docs to be built locally much easier as well.