The Implicit Classes guide should be part of the Scala Book

We would like the [Scala Book](https://docs.scala-lang.org/scala3/book/introduction.html) to be a complete documentation of the language.

The guide [Implicit Classes](https://docs.scala-lang.org/overviews/core/implicit-classes.html) should be merged into the Scala Book. I think we should move it into the Contextual Abstractions chapter, just after the page Implicit Conversions.