Document experimental Scala 3 support (#30849)

Perhaps we should add a page to the docs summarizing the status
of Scala 3 support, so we can point people to that rather than directly
at the GitHub issues?

akka-docs/src/main/paradox/project/index.md

@@ -5,6 +5,7 @@
 @@@ index
 
 * [../common/binary-compatibility-rules](../common/binary-compatibility-rules.md)
+* [scala3](scala3.md)
 * [downstream-upgrade-strategy](downstream-upgrade-strategy.md)
 * [../common/may-change](../common/may-change.md)
 * [ide] (../additional/ide.md)

akka-docs/src/main/paradox/project/scala3.md

@@ -0,0 +1,21 @@
+# Scala 3 support
+
+Akka has experimental support for Scala 3.
+
+## Using 2.13 artifacts in Scala 3
+
+You can use [CrossVersion.for3Use2_13](https://scala-lang.org/blog/2021/04/08/scala-3-in-sbt.html#using-scala-213-libraries-in-scala-3)
+to use the regular 2.13 Akka artifacts in a Scala 3 project. This has been
+shown to be successful for Streams, HTTP and gRPC-heavy applications.
+
+## Scala 3 artifacts
+
+Starting with Akka version 2.6.18 (and on current [development snapshots](https://oss.sonatype.org/content/repositories/snapshots/com/typesafe/akka/akka-actor_3/)),
+we are publishing experimental Scala 3 artifacts that can be used 'directly'
+(without `CrossVersion`) with Scala 3.1.1-RC1. The 2.6.17 artifacts can be used only with nightly builds of Scala 3 (i.e. `3.1.1-RC1-bin-20211007-c041327-NIGHTLY`).
+
+There are some known run-time problems with these artifacts still, which are
+tracked under the umbrella issue [#30242](https://github.com/akka/akka/issues/30242).
+We encourage you to try out these artifacts and report any findings.
+
+We do not promise @ref:[binary compatibility](../common/binary-compatibility-rules.md) for these artifacts yet.