WiP List of all experimental features

[tgodzik]

As a conclusion of #21505 we first want a list of all currently experimental features. We will later move it to another easily findable place.

[tgodzik]

One thing is experimental features here: https://github.com/scala/scala3/blob/21a3d393cbac36447ad7a79a600ef98ed15525b6/docs/_docs/reference/experimental/overview.md

But I guess it's not up to date, maybe best to update that then as a first step?