Switch docs to a better theme

[esden]

We need a better theme for the documentation.

Furo ticks pretty much all the boxes:

• dark/light theme switch with the ability to follow OS setting
• source code link button
• edit page button (linking to the github repo)
• good navigation on mobile
• Looks good on mobile, good quality responsive design
• Code blocks provide a copy icon for easy reuse

Now we "just" need to switch over to it.

We might want to tweak the colors to make it our own though a little bit.