Make doc working again

[Odomontois]

After #1192 , Unidos apparently is trying to compile Scala 3 sources using the Scala 2 compiler during doc generation.
This results in numerous syntax error such as

[error] /.../tofu/modules/kernel/src/main/scala-3/tofu/ScopedInstancesMacro.scala:7:27: ';' expected but ':' found.
[error] trait ScopedInstancesMacro:
[error]                           ^
[error] /...tofu/modules/kernel/src/main/scala-3/tofu/ScopedInstancesMacro.scala:15:12: expected start of definition
[error]   implicit inline def makeExecuteCE3[Tag, F[_]](p1: ExecutionContext): ScopedExecute[Tag, F] =
[error]            ^
[error] /.../tofu/modules/kernel/src/main/scala-3/tofu/concurrent/AtomInstances.scala:8:9: type application is not allowed for postfix operators
[error]   given representableKInstance[A]: RepresentableK[({ type L[x[_]] = Atom[x, A] })#L] =
[error]         ^

We need to exclude somehow scala 3 sources from unidoc