Switch to mkdocs material theme for documentation

[browniebroke]

Description

Explore replacing our theme by mkdocs-material.

Light theme:

Dark theme:

Some things I'm not happy about:

• I think the colors for the dark theme aren't quite there yet (especially the contrast of the links).
• Syntax highlighting for code snippets is lost unless we switch to code fences (no idea how it works now)
• This theme has a lot of features, there might be more to enable