Documentation for the Scala 2.13-3.x sandwich

[smarter]

Currently the behavior of the Scala 2<->3 dependencies and then new methods such as CrossVersion.for3Use2_13 are only documented in the release notes: https://eed3si9n.com/sbt-1.5.0-beta, ideally they should be part of https://www.scala-sbt.org/1.x/docs/Cross-Build.html so we have one complete resource we can point people to /cc @adpi2