Move docs dependencies into their own group

[briantist]

Now that we have poetry groups set up, with optional dev dependencies in their own group, I think we can further split off docs dependencies into their own group.

The reasons I want to do this:

• encourage more documentation contributions and make it easier for minimal testing of them locally
• to clarify dependencies for readthedocs (RTD)
• to aid in dependency installation for first draft PR docs build workflow #1042
• to give an option not to install these for folks who only want to run pytest locally
• make maintenance of these dependencies easier when we know they don't conflict with other dev dependencies
• to clarify dependencies around doctest

That last point is where I think we're going to hit some snags. Doctest is used to test code examples in the documentation, and we've done some weird things with it. It's a little hard to pick apart right now, and I think that stuff needs maintenance. It would be helpful for us to figure out what the future is for doctest, whether we want to lean into it and shore it up, or remove it (and its dependencies).