Try pydata-sphinx-theme in docs

[TomNicholas]

• [x] Closes #8701
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst

How might we want to move headers/sections around to take advantage of now having a navigation bar at the top? Adding an explicit link to the tutorial.xarray.dev site would be good.

[TomNicholas]

Docs build failed because we fail on warnings:

preparing documents... WARNING: unsupported theme option 'repository_url' given
WARNING: unsupported theme option 'repository_branch' given
WARNING: unsupported theme option 'path_to_docs' given
WARNING: unsupported theme option 'use_repository_button' given
WARNING: unsupported theme option 'use_issues_button' given
WARNING: unsupported theme option 'home_page_in_toc' given
WARNING: unsupported theme option 'extra_footer' given