[Javadoc] Ignore Kotlin samples (#3473)

* [Javadoc] Ignore Kotlin samples

This is a laconic solution to avoid an error message (#3466) for Kotlin samples in unstable Javadoc format. There are 2 problems.
1. Javadoc's content model does not contain samples links at all. It should contain something like `samplesSectionContent` in the base plugin.
2. Sample Transformer from the base plugin knows nothing about Javadoc's content model. see  `DefaultSamplesTransformer.dfs`, e.g. `JavadocContentGroup` is unavailable from the base plugin.

(cherry picked from commit e502b2c)

dokka-subprojects/plugin-base/src/main/kotlin/org/jetbrains/dokka/base/transformers/pages/DefaultSamplesTransformer.kt

@@ -19,6 +19,9 @@ import org.jetbrains.dokka.analysis.kotlin.sample.SampleSnippet
 
 internal const val KOTLIN_PLAYGROUND_SCRIPT = "https://unpkg.com/kotlin-playground@1/dist/playground.min.js"
 
+/**
+ * It works ONLY with a content model from the base plugin.
+ */
 internal class DefaultSamplesTransformer(val context: DokkaContext) : PageTransformer {
 
     private val sampleAnalysisEnvironment: SampleAnalysisEnvironmentCreator =

dokka-subprojects/plugin-javadoc/src/main/kotlin/org/jetbrains/dokka/javadoc/JavadocPlugin.kt

@@ -94,6 +94,13 @@ public class JavadocPlugin : DokkaPlugin() {
         javadocPreprocessors with DeprecatedPageCreator order { before(rootCreator) }
     }
 
+    // defaultSamplesTransformer knows nothing about Javadoc's content model
+    internal val emptySampleTransformer: Extension<PageTransformer, *, *> by extending {
+        CoreExtensions.pageTransformer providing {
+            PageTransformer { it }
+        } override dokkaBasePlugin.defaultSamplesTransformer
+    }
+
     internal val alphaVersionNotifier by extending {
         CoreExtensions.postActions providing { ctx ->
             PostAction {

dokka-subprojects/plugin-javadoc/src/test/kotlin/org/jetbrains/dokka/javadoc/JavadocClasslikeTemplateMapTest.kt

@@ -42,6 +42,32 @@ internal class JavadocClasslikeTemplateMapTest : AbstractJavadocTemplateMapTest(
         }
     }
 
+    @Test
+    fun `single class should not render Kotlin sample`() {
+        dualTestTemplateMapInline(
+            kotlin =
+            """
+            /src/source0.kt
+            /**
+             * some doc
+             * @sample [sample]
+             */
+            class Test {
+            }
+            
+            fun sample(){
+                val a = 0
+            }
+            """
+        ) {
+            assertEquals(0, context.logger.errorsCount)
+            assertEquals(0, context.logger.warningsCount)
+            val map = allPagesOfType<JavadocClasslikePageNode>().first{ it.name == "Test" }.templateMap
+            assertEquals("Test", map["name"])
+            assertEquals("<p>some doc</p>", map["classlikeDocumentation"])
+        }
+    }
+
     @Test
     fun `single function`() {
         dualTestTemplateMapInline(