add note about disabling doc generation

[tpolecat]

The SBT setup page should include a note somewhere about disabling doc generation for Scala 3 because it doesn't quite work and it will hit 100% of library authors trying to publish.

  Compile / doc / sources := {
    val old = (Compile / doc / sources).value
    if (isDotty.value)
      Seq()
    else
      old
  }