Add titlesonly to reference / topics top-level TOCs

This prevents duplicated headings in places where the second-level index page has intro blurbs for each subsection (and also means we're not listing out long multi-section pages in full in the index, but given how big these indexes are that's probably not a bad thing).

docs/reference/index.rst

@@ -4,6 +4,7 @@ Reference
 
 .. toctree::
     :maxdepth: 2
+    :titlesonly:
 
     pages/index
     streamfield/index

docs/topics/index.md

@@ -3,6 +3,7 @@
 ```eval_rst
 .. toctree::
     :maxdepth: 2
+    :titlesonly:
 
     pages
     writing_templates