Turn off dark mode default in docs

[canyon289]

Describe the bug
The docs don't look good in dark mode. Is there some setting to ensure it defaults to light mode initially?

To Reproduce
I'm not sure but I'm thinking its reading from my system or browser setting

Expected behavior
Light mode always

Additional context
Versions of arviz and other libraries used, operating system used, and anything else that may be useful.

[networkpsych]

pydata/pydata-sphinx-theme#458

The same question was issued here, and the commenter mentioned changing the html_context for the sphinx setup:

html_context = {
   # other contexts
   "default_mode": "light"
}

[OriolAbril]

We should pin the pydata-sphinx-theme to <0.9.0 in https://github.com/arviz-devs/arviz/blob/main/requirements-docs.txt.

The introduction of the dark theme (which defaults to whatever theme your browser is using but can be changed as pointed out by @Autoclitic and as explained in the theme docs) is quite secondary imo. The version has also changed how the layout works, so our trick from https://github.com/arviz-devs/arviz/blob/main/doc/_templates/layout.html no longer works, both sidebars appear and overlay with the content of the homepage which looks awful in both modes.

I think we should pin the theme (urgent) and then whenever we can open a PR to update the layout trick to remove all sidebars from the homepage (not urgent)