DOCS: temporary pin pydata-sphinx-theme to 0.13

The pydata-sphinx-theme has a new release, and that causes some issues with the top bar:

![image](https://github.com/pandas-dev/pandas/assets/1020496/c0f2a1ff-c2a4-46e8-9ed9-6ab43316c20c)

So we can maybe temporarily pin the theme until that is fixed
(labeling as 2.1.1, because we should ensure to use the previous theme in the release branch in case we don't fix this by the time of the 2.1.1 release) thanks @jorisvandenbossche 