qdoc: Write contents elements to index file for collections

When the CollectionNode class was changed from an Aggregate to a LeafNode,
its output to the index file was not updated. This change ensures that the
contents elements in collection nodes are output to the index file correctly.

Change-Id: Id19a1c974c10d54f6ff1d3499726bbfa1b98beda
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>