Pin to jinja2<3.1 to avoid Readthedocs build error

[GenevieveBuckley]

Closes issue #277

[GenevieveBuckley]

Note: I didn't test this locally before opening this PR. I'm hoping the CI checks will run the Readthedocs build with the updated yaml file and test it for us.

Ordinarily I'd test any changes to yaml environment files locally, just to check the dependencies can resolve. But I wasn't able to do that this time, because on my M1 Mac conda can't create even the current environment-doc.yml

Encountered problems while solving:
  - nothing provides msgpack-python <1.0.0 needed by distributed-2.10.0-py_0

I think that's just an M1 Mac problem, so it probably won't matter since the CI uses linux.

@jakirkham you did a lot of work on #273 so you're the most expert on the documentation CI setup right now - please let me know if you think I should be doing anything differently

[jakirkham]

Sorry for the slow reply Genevieve 😞

Thanks for digging into this issue! 🙏

Remember some weird things going on with Sphinx, but have forgotten what they were now. Am wondering if maybe this was a dependency issue that got cleared up somewhere? Unfortunately search is failing me atm

[GenevieveBuckley]

All the CI checks are passing, so I'm going to merge this. Fingers crossed it resolves the problems.