Use sbt-paradox-apidoc plugin (#26660)

Instead of using custom code here. Checked with 'meld' that the result
is the same, except for problems that were already there (filed #26659).

build.sbt

@@ -270,6 +270,7 @@ lazy val docs = akkaModule("akka-docs")
     ),
     Compile / paradoxGroups := Map("Language" -> Seq("Scala", "Java")),
     resolvers += Resolver.jcenterRepo,
+    apidocRootPackage := "akka",
     deployRsyncArtifact := List((paradox in Compile).value -> s"www/docs/akka/${version.value}")
   )
   .enablePlugins(