language tour: improve Traits page

[SethTisue]

fixes #3097

[SethTisue]

I'll have to figure out how to get the Scala-3-only example past mdoc — probably by making Scala 3 the default language version for mdoc.

[bishabosha]

I had an exploration of switching mdoc to scala 3 - basically a lot of localized pages either need updating or mdoc turned off. See here: main...bishabosha:docs.scala-lang:mdoc-use-scala3

My process was iterate running scripts/run-mdoc.sh, fix errors, and check for more errors, which could end up being a large time sink

[SethTisue]

@bishabosha I think we could reasonably just disable mdoc on the localized pages; many of them are quite out of date anyway

for any particular language, if a motivated volunteer wanted to do the work necessary to reenable mdoc for that language's pages, they could do so; we could open a "help wanted" issue on it

and/or could we have a magic comment or magic metadata on the top of certain pages to mark them as needing to be run with Scala 2 instead?