Move from mkdocs to docusaurus

[fretchen]

This PR should allow us to move from mkdocs to docusaurus. The content changes are minimal (close to zero).

• mostly had to change config files like package.json and mkdocs.yml
• Had to add a component on the docs/index.mdx
• Had to update the syntax slightly of the admonition

No real content changes otherwise. A live preview of the resulting website can be found here.