Set navigation_with_keys=False explicitly

[scottyhq]

Currently this warning is raised:

The default value for navigation_with_keys will change to False in the next release. If you wish to preserve the old behavior for your site, set navigation_with_keys=True in the html_theme_options dict in your conf.py file. Be aware that navigation_with_keys = True has negative accessibility implications: https://github.com/pydata/pydata-sphinx-theme/issues/1492

Can pre-empt this change by changing the config: https://github.com/xarray-contrib/xarray-tutorial/blob/main/_config.yml#L62

sphinx:
  config:
    html_theme_options:
      navigation_with_keys: false