Latest docs must build from `main` (not `master`)

[bittner]

As can be seen on RTD, the "latest" docs are configured to build from the master branch. However, we switched the default branch to main with #204 half a year ago. We apparently overlooked that updating the RTD configuration settings was necessary, too.

@jcassee Can you update the value on RTD, please, and trigger a build of the latest docs?

[jcassee]

Anyway, I set it to "main".

[jcassee]

I also made @bittner maintainer at RTD. Thanks for noticing the problem.

[bittner]

Great! Thanks, Joost! 🥇