[Docs] Fix wrong syntax using paradox directives (#29876)

* Fix wrong syntax using paradox directives
* Simplify collection.md - Only available for Scala, so hide most of the page for Java

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

@@ -3,7 +3,7 @@ project.description: Data immutability using Project Lombok
 ---
 # Immutability using Lombok
 
-A preferred best practise in Akka is to have immutable messages. Scala provides case class which makes it extremely easy
+A preferred best practice in Akka is to have immutable messages. Scala provides case class which makes it extremely easy
 to have short and clean classes for creating immutable objects, but no such facility is easily available in Java. We can make use
 of several third party libraries which help is achieving this. One good example is Lombok.
 
@@ -17,28 +17,16 @@ Lombok handles the following details for you. It:
 * creates correct `equals`, `hashCode` and a human-friendly `toString`
 * creates a constructor requiring all fields.
 
-
 ### Adding Lombok to your project
+
 To add Lombok to a Maven project, declare it as a simple dependency:
 
-Maven
+@@dependency[Maven,Gradle] {
-: @@dependency {
+  group="org.projectlombok"
-    <dependency>
+  artifact="lombok"
-        <groupId>org.projectlombok</groupId>
+  version=1.18.10
-        <artifactId>lombok</artifactId>
-        <version>1.18.10</version>
-    </dependency>
 }
 
-Gradle
-: @@dependency {
-    dependencies {
-    	compileOnly 'org.projectlombok:lombok:1.18.10'
-    	annotationProcessor 'org.projectlombok:lombok:1.18.10'
-    }
-} 
-    
-    
 # Using lombok
 
     @Value
@@ -82,6 +70,8 @@ getter, toString, hashCode, equals.
     }
 
 ### Integrating Lombok with an IDE
+
 Lombok integrates with popular IDEs:
+
 * To use Lombok in IntelliJ IDEA you'll need the [Lombok Plugin for IntelliJ IDEA](https://plugins.jetbrains.com/plugin/6317-lombok) and you'll also need to enable Annotation Processing (`Settings / Build,Execution,Deployment / Compiler / Annotation Processors` and tick `Enable annotation processing`)
 * To Use Lombok in Eclipse, run `java -jar lombok.jar` (see the video at [Project Lombok](https://projectlombok.org/)).

akka-docs/src/main/paradox/stream/operators/Sink/collection.md

@@ -4,14 +4,15 @@
 
 @ref[Sink operators](../index.md#sink-operators)
 
+@@@div { .group-scala }
+
 ## Signature
 
 @apidoc[Sink.collection](Sink$) { scala="#collection[T,That](implicitcbf:akka.util.ccompat.Factory[T,Thatwithscala.collection.immutable.Iterable[_]]):akka.stream.scaladsl.Sink[T,scala.concurrent.Future[That]]" }
 
-
 ## Description
 
-@scala[Collect values emitted from the stream into an arbitrary collection `That`. The resulting collection is available through a `Future[That]` or when the stream completes. Note that the collection boundaries are those defined in the `CanBuildFrom` associated with the chosen collection. See [The Architecture of Scala 2.13's Collections](https://docs.scala-lang.org/overviews/core/architecture-of-scala-213-collections.html) for more info. The [`seq`](seq.html) operator is a shorthand for `Sink.collection[T, Vector[T]]`.]@java[Operator only available in the Scala API. The closest operator in the Java API is [`Sink.seq`](seq.html).]
+Collect values emitted from the stream into an arbitrary collection `That`. The resulting collection is available through a `Future[That]` or when the stream completes. Note that the collection boundaries are those defined in the `CanBuildFrom` associated with the chosen collection. See [The Architecture of Scala 2.13's Collections](https://docs.scala-lang.org/overviews/core/architecture-of-scala-213-collections.html) for more info. The [`seq`](seq.html) operator is a shorthand for `Sink.collection[T, Vector[T]]`.
 
 ## Example
 
@@ -22,10 +23,10 @@ Scala
 
 ## Reactive Streams semantics
 
-@@@
+@@@@div { .callout }
-
-@@@div { .group-scala .callout }
 
 **cancels** If too many values are collected
 
+@@@@
+
 @@@

akka-docs/src/main/paradox/stream/operators/Source/never.md

@@ -8,9 +8,7 @@ Never emit any elements, never complete and never fail.
 
 ## Signature
 
-@@signature [Source.scala](/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala) { #never }
+@apidoc[Source.never](Source$) { scala="#never[T]:akka.stream.scaladsl.Source[T,akka.NotUsed]" java="#never()" }
-
-@@@
 
 ## Description