.scala-steward.conf documentation should be machine-checked

[fthomas]

It would be nice if the .scala-steward.conf documentation would be created and verified by mdoc so that the examples in that document are decoded correctly. This will get more important if we increase the visibility of that documentation (#1340).

[fthomas]

I'm currently experimenting with this here: https://github.com/fthomas/scala-steward/compare/wip/checked-doc

[fthomas]

#3103 is a first step in this direction.