Remove source files in _scala3-reference/

[julienrf]

Initially, the Dotty documentation used to be published on the website https://dotty.epfl.ch. When Dotty became Scala 3.0.0, it was natural to move its documentation here, on the website https://docs.scala-lang.org (see #2112). However, by doing so we realized that the compiler team lost the ability to implement something in the compiler and to document it in a single pull request. They now had to implement their features in the dotty repo, and to document these features here.

I believe the Scala 3 reference documentation should be hosted on the website docs.scala-lang.org, but I also understand that it is more convenient for the compiler team if they can implement things and document their work within a single repo. Consequently, we decided with VirtusLab to produce the HTML pages of the Scala 3 reference documentation from source files hosted on the dotty repo. These pages are published at the URL https://docs.scala-lang.org/scala3/reference, so they “shadow” any content produced by the docs.scala-lang repo. Therefore, we should remove the duplicated source files from this repo.

[bishabosha]

some links need updating

[bishabosha]

one more

Ran on 1090 files!
- ./_site/scala3/book/ca-summary.html
  *  External link https://docs.scala-lang.org/scala3/reference/contextual.html failed: 404 No error