Also generate Scala 3 scaladoc (requires tasty files)

[mkurz]

Scala 3 scaladoc ("dottydoc") does not support .scala files at the moment (unless Scala 2 scaladoc which does). It requires .tasty and/or .jar files to generate the scala doc from. That means, when on Scala 3, we need to trigger a full compilation to generate those .tasty files.
See

• https://github.com/sbt/sbt/blob/v1.9.2/main/src/main/scala/sbt/Defaults.scala#L2161
• scaladoc tool : argument files (@-files)  scala/scala3#11454 (comment)
• Fix typo: suports -> supports scala/scala3#18215

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However, currently we don't really support multiple api docs on the Play Framework website. When you click on "Scala" next to "Browse APIs" you get taken to /api/scala/index.html (like not /scala_2.13/ or similiar). That means we probably want to modify the website to have a "Scala 2" and "Scala 3" link. It's also obvious from the releasing docs that we currently only copy over one scaladoc version to the generated docs (See "Step 3 - Release omnidoc")

To be honest, I am not sure if this omnidoc thing is a good thing for long term. We already discussed we should at some point change the documentation and website setup. I think when doing that we should get rid of omnidoc and find another way to publish (api) docs, e.g. let each project just generate and push it's scaladoc to a github folder itself instead of gathering it at one place like omnidoc does. Also it's quite some sbt kung-fu...

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Note for myself so I don't forget for next time in case I ran into troubles again:
The scaladoc will be generated in target/scala-XXX/omnidoc/scaladoc/
Using sbt ++X.X --debug scaladoc displays all the arguments that get passed to the scaladoc command, which makes it quite easy to rebuild the command ourselves to run it standalone for testing. (I also had to use -mem 4096 because of out of memory a couple of times with Scala 3).