Improve site style: logo, font, formatting
- Font: Roboto Mono https://squidfunk.github.io/mkdocs-material/setup/changing-the-fonts/
- Theme logo: Bundled icon https://squidfunk.github.io/mkdocs-material/setup/changing-the-logo-and-icons/
- Light/Dark switcher logo: Bulb
- Enable more MkDocs extensions/plugins