Make community build docs experiment-aware

[abgruszecki]

No description provided.

[abgruszecki]

Actually tested in #11936.

[abgruszecki]

Ok, this seems to work as intended, the parts of the tests that fail are the ones that fail on master as well.

[abgruszecki]

@BarkingBad Krzysztof mentioned that you'll be looking at the CI problems with the doctool, correct? Could you ping me when you're done so that I can rebase this PR?

[BarkingBad]

Hey @abgruszecki, I think it was becuase of this nasty bug #11948.

Becuase of that, the useScaladoc.value in dotty/sbt-dotty/sbt-test/sbt-dotty/scaladoc/build.sbt was returning false (becuase version is 3.0.1), hence the aseertion was failing without meaningful comment.

[BarkingBad]

Oh I didn't see you already fixed that. I think you can rebase now to make the CI pass. You can close my PR or I can resolve the conflict just to add that assertion message for future debugging.

[abgruszecki]

@odersky this PR adds support for compilerSupportExperimental in the doc part of the community build. You can see in #11936 that the doctool community build passes if we simulate an experimental-unsupporting build. There is a single compiler test that fails when we do that, but solving that is a separate issue:

9 suites passed, 1 failed, 10 total
    tests/plugins/neg/divideZero-research failed

[odersky]

I am a bit surprised that so many projects require experimental. How did they build before?

I would be good if @nicolasstucki or @liufengyun could review as well.