Make alerts (admonition) pretty
Some links for inspiration:
- https://squidfunk.github.io/mkdocs-material/extensions/admonition/
- https://github.com/Python-Markdown/markdown/blob/master/markdown/extensions/admonition.py
- https://github.com/nanoc/nanoc.ws/blob/master/content/assets/style/_admonition.scss
- https://github.com/squidfunk/mkdocs-material/blob/master/src/assets/stylesheets/extensions/_admonition.scss