Add generic tuples documentation

[nicolasstucki]

We should have a page in dotty.epfl.ch describing the generic Tuple and provide some examples.

[SethTisue]

I was trying to find this the other day and now I know why I didn't find it

one thing we do have is https://www.scala-lang.org/2021/02/26/tuples-bring-generic-programming-to-scala-3.html