Add support for docutils==0.20.x

[humitos]

Note that docutils==0.20.x is supported on Sphinx>=7.0.1

Closes #1516
Closes #1323
Closes #1336

[humitos]

I did the same visual QA test as in #1336 (comment) but using only Sphinx==7.2.x and I didn't find any visual difference.

Besides, we can see the changes in the HTML are pretty basic: https://gist.github.com/humitos/11dac3efa0ed0bca4bb4d06f5facc1f2

This PR is another good candidate for 2.x release -- cc @agjohnson

[agjohnson]

We talked a bit about going straight to 0.20 support on our 2.0rc1 candidate, we're still figuring out what is best here.

But testing this against previous docutils and the output is largely unchanged. I only noticed a different between a couple section headings between docutils 0.19 and 0.20, but 0.20 fixes what seem to be regressions in docutils 0.19.

[humitos]

I'm happy to move forward here and do a 2.0rc1, in particular if 0.19 has some regressions. There is no need to expose a version of our theme with docutils 0.19 if we already know they are fixed in 0.20.

I'm going to merge this PR so we can do a "rc" release today or tomorrow.

If you disagree with this decision, I'm happy to revert it 😄