Make documentation for Scala 3

[FabioPinheiro]

The documentation and tutorials needs to be reviewed to account for the new Scala 3 version.

[sjrd]

To anyone coming here before we adapt anything, please note that:

• Take any Scala.js documentation, and set scalaVersion := "3.0.0"
• You may have to adapt libraryDependencies, just like in any Scala project.
• That's it.

There is no difference in how one uses Scala.js in Scala 3 compared to Scala 2.

[sjrd]

The main tutorial has been using Scala 3 for a while now.