Docs: Don't load jQuery from 3rd party CDN for v4 Docs

Created by: coliff

Suggestion: on the v4 docs jQuery is loaded from the jQuery CDN (https://code.jquery.com/jquery-3.5.1.slim.min.js), with a local fallback. It'd be less HTML code and likely always better for performance if it always loaded jQuery locally. It'll save the cost of opening a new TCP connection (DNS resolution, TCP handshake and TLS negotiation). Browsers don't have Cross-Domain caching so there's no advantage in using a 3rd party CDN anymore (as your site is served from a CDN anyway). Good explainer here (with Bootstrap being used as the example): https://csswizardry.com/2019/05/self-host-your-static-assets/