switch from tut to mdoc (for checking code snippets)

[SethTisue]

we use tut (https://github.com/tpolecat/tut, http://tpolecat.github.io/tut/) to actually run code snippets and make sure they compile and run and give the expected result. this happens during PR validation

tut is considered deprecated and the author (Rob Norris / @tpolecat) suggests everyone switch to @olafurpg's mdoc (https://github.com/scalameta/mdoc), which didn't exist yet at the time we adopted tut in this repo

so that's a possible todo at some point. when? well for one thing, it seems unlikely tut will add Scala 3 support.

[SethTisue]

mdoc support for Scala 3 is in progress, or soon to be in progress anyway: scalameta/mdoc#347