Scaladoc: fix generation of unique header ids (scala#22779)

This PR fixes the generation of unique header id's.

Previously, the id's were deduplicated globally, now they are
deduplicated only within one page.
Custom logic for that was removed, we now rely on `HeaderIdGenerator`
from flexmark.
[Cherry-picked 8734fbd]

Author: Florian3k

scaladoc/src/dotty/tools/scaladoc/tasty/comments/markdown/SectionRenderingExtension.scala

@@ -26,21 +26,17 @@ object SectionRenderingExtension extends HtmlRenderer.HtmlRendererExtension:
   def rendererOptions(opt: MutableDataHolder): Unit = ()
 
   case class AnchorLink(link: String) extends BlankLine(BasedSequence.EmptyBasedSequence())
-  object SectionHandler extends CustomNodeRenderer[Section]:
-    val repeatedIds: mutable.Map[(NodeRendererContext, String), Int] = mutable.Map()
+  class SectionHandler extends CustomNodeRenderer[Section]:
     val idGenerator = new HeaderIdGenerator.Factory().create()
+    idGenerator.setResolveDupes(true)
     override def render(node: Section, c: NodeRendererContext, html: HtmlWriter): Unit =
       val Section(header, body) = node
-      val headerText = header.getText.toString
-      val idSuffix = repeatedIds.getOrElseUpdate((c, headerText), 0)
-      val ifSuffixStr = if(idSuffix == 0) then "" else idSuffix.toString
-      repeatedIds.update((c, headerText), idSuffix + 1)
 
       /* #19524 flexmark's `HeaderIdGenerator` does not appear to be thread-safe,
        * so we protect its usage with a full `synchronize`.
        */
       val id = idGenerator.synchronized {
-        idGenerator.getId(headerText + ifSuffixStr)
+        idGenerator.getId(header.getText)
       }
 
       val anchor = AnchorLink(s"#$id")
@@ -68,13 +64,12 @@ object SectionRenderingExtension extends HtmlRenderer.HtmlRendererExtension:
   object Render extends NodeRenderer:
     override def getNodeRenderingHandlers: JSet[NodeRenderingHandler[_]] =
       JSet(
-        new NodeRenderingHandler(classOf[Section], SectionHandler),
+        new NodeRenderingHandler(classOf[Section], new SectionHandler),
         new NodeRenderingHandler(classOf[AnchorLink], AnchorLinkHandler)
       )
 
   object Factory extends NodeRendererFactory:
     override def apply(options: DataHolder): NodeRenderer = Render
 
-
   def extend(htmlRendererBuilder: HtmlRenderer.Builder, tpe: String): Unit =
     htmlRendererBuilder.nodeRendererFactory(Factory)