Upgrade to Docusaurus v3

[gingerwizard]

This is needed not least to address security issues in dependencies.

Number of tasks:

• Use .mdx extension whenever we use JSX, import, or export. In future versions of Docusaurus, .md files will be parsed as standard CommonMark.
• MDX3 is stricter, address compilation issues - common ones are here. Run npx docusaurus-mdx-checker
• Upgrade all packages
• Upgrade node to v18/v20 LTS
• Upgrade to React v18.0+, MDX v3.0+, TypeScript v5.1+, prism-react-renderer v2.0+, react-live v4.0+, remark-emoji v4.0+, mermaid v10.4+
• Admonian colors have changed - specifically warning

As a result of possible visual changes, we will implement visual testing first. We plan to use this moving forward for all package upgrades.

• Implement visual testing - this is needed for all future package dependencies.

[gingerwizard]

Done!