Introduce Transformer for Content Menus

This reintroduces the contents directive which was destroyed by switching to transformers

Author: linawolf

packages/guides/src/Compiler/NodeTransformers/ContentMenuNodeWithSectionEntryTransformer.php

@@ -14,6 +14,7 @@
 use phpDocumentor\Guides\Nodes\Node;
 
 use function assert;
+use function count;
 
 use const PHP_INT_MAX;
 

packages/guides/src/Nodes/DocumentNode.php

@@ -246,7 +246,7 @@ public function getFootnoteTargetAnonymous(): FootnoteTarget|null
         return null;
     }
 
-    public function getDocumentEntry(): DocumentEntryNode|null
+    public function getDocumentEntry(): DocumentEntryNode
     {
         if ($this->documentEntry === null) {
             throw new Exception('DocumentEntry may not be accessed before initialization');