Upgrade to docusorus v2 for Scala 2/3 code samples

[Julien Truffaut]

Docusaurus v2 lets us define code samples in tabs, so we could have one tab for Scala 2 and one for Scala 3.

https://v2.docusaurus.io/docs/markdown-features#customizing-tabs

[Yilin Wei]

I will probably try using Laika for this.

[Arman Bilge]

Yes, Laika supports using tabs, see for example the Toolkit docs.

https://typelevel.org/toolkit/#example