Apply Wagtail design system to the docs

[tbrlpld]

Figma community version of the Wagtail design system: https://www.figma.com/community/file/1098619062654073532

Now that there is a full design system, I think it would be great to apply the styles defined in there to the docs.

I just dropping this here as a reminder. We probably want to update existing tickets to incorporate that information and add new ones where appropriate.

[lb-]

Some progress has been made for this via
#194