Switch to new design with PyData theme

[lexming]

Preview: https://vscdocumentationbeta.readthedocs.io
Changelog: tasks.md
Internal docs: README + Style guide
Fixes: #329

⚠️ The purpose of this PR is not to receive any review, it just allows a one button push to move the new design into production when the date of the switch is settled.

Once this PR is merged, we have to manually disable the versioning from the URLs (i.e. the /en/latest part). Whoever has access to our account in https://readthedocs.org/dashboard, please make the following changes to the configuration:

1. Go to Admin > Advanced Settings and enable Single version

2. Go to Admin > Redirects and click Add redirect

   • Select as Redirect Type: Prefix redirect
   • Define From URL as: /en/latest/
   • Click Save

   Old links will continue to work with this redirect.