Add shiki-twoslash tooltips to docsite

[cspotcode]

For example, get fancy IntelliSense tooltips in your code samples:
https://mael.dev/clipanion/docs/getting-started

For us, a unique challenge: we must also render down to README.md. Might need a processing step in our README generator to strip shiki twoslash syntax.

https://github.com/shikijs/twoslash/tree/main/packages/docusaurus-preset-shiki-twoslash
https://github.com/shikijs/twoslash

[cspotcode]

Implemented and closed by #1645