fix(shell-docs): drop section headers from per-framework override nav

Per-framework meta.json files (mastra, langgraph, llamaindex, etc.)
mirror the root tree's section names ("Getting Started", "Basics").
Passing them through buildFrameworkOverridesNav into the merged
sidebar caused duplicate React keys when the merge ran — every root
section collided with the override's copy of the same title.

The override block is already wrapped in a single "{frameworkName}"
section by mergeFrameworkNav, so nested section headers added no
information anyway. Drop them at the filter step.

Author: samjulien

showcase/shell-docs/src/lib/docs-render.tsx

@@ -348,9 +348,13 @@ export function buildFrameworkOverridesNav(framework: string): NavNode[] {
       if (children.length > 0) {
         filtered.push({ ...node, children });
       }
-    } else {
-      filtered.push(node);
     }
+    // Intentionally drop section nodes. Per-framework meta.json files
+    // tend to mirror the root tree's sections ("Getting Started",
+    // "Basics", etc.) and flowing them through here would (a) collide
+    // with root sections of the same name on React keys and (b) double
+    // up the visual hierarchy — the override block is already wrapped
+    // in a single `{frameworkName}` section by mergeFrameworkNav.
   }
   return filtered;
 }