DOC: use new pydata-sphinx-theme name

[jorisvandenbossche]

Following pydata/pydata-sphinx-theme#102, need to use the new package name

We could actually also start using the released version. But going to merge this soon when CI passes (as I think doc build in other PRs will start to fail)

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 1.0.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 bfa29f10d4115fada208848e6e7756040525e42d

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #32840: DOC: use new pydata-sphinx-theme name'

4. Push to a named branch :

git push YOURFORK 1.0.x:auto-backport-of-pr-32840-on-1.0.x

5. Create a PR against branch 1.0.x, I would have named this PR:

"Backport PR #32840 on branch 1.0.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.