Add support for Scaladoc 3

[jodersky]

This adds support for docJar generation under Scala 3.0.0-RC1, which is currently broken[1].

I've tried to keep backwards compatibility as much as I could. However, there is one sort-of-breaking change with this: docJar now has a dependency on the compile task (because Scaladoc 3 uses tasty files, not source files anymore). This dependency prevents generating documentation for projects that would previously not compile but could still generate documentation.

[1]: See the discussion on Gitter if you're interested, but the gist of it is that Scaladoc 3 is really not compatible with dottydoc, yet dottydoc is no longer available for Scala >= 3.0.0-RC1.

[jodersky]

FYI: the integration test failure of BetterFiles seems unrelated. It currently also fails on my machine under mill master (c500ca9).

[lefou]

I'm pretty sure I review and commented before, but I can't find these comment anymore. Strange. I'll try to review soon again. Thanks for the PR.

[lefou]

I'm pretty sure I review and commented before, but I can't find these comment anymore. Strange. I'll try to review soon again. Thanks for the PR.

I'm sorry, I mixed up PRs. There is already a PR for scaladoc 3 #1154, on which I commented.

[jodersky]

oh, no! How did I miss this? Let me also take a look at the other one

[lefou]

Looks good to me.

[lefou]

I'm in favor merging this PR despite the fact that we have another older PR #1154 . This one handles doc resources better and comes with additional tests.

Unrelated to merging this one, I think we should better document all these different isDotty, isScala3, isScala3Milestone util method to better understand why and when to use them.

Thanks for this PR!

[hmf]

I would like to know if static site generation was tested. The API is generated correctly, However the static site files are just copied over to the destination directory. I may have an error on my side but just checking.
TIA

EDIT: I forced an incorrect template name and an error is generated. So it seems like static site generatton is partially working. However, no files are converted (templates or md).