Add <type> section to indexes.

Author: tabatkins

indexes/Overview.bs

@@ -7,6 +7,7 @@ Status: NOTE
 Work Status: exploring
 Editor: Tab Atkins-Bittner
 ED: https://drafts.csswg.org/indexes/
+TR: ---
 Abstract: A listing of every term defined by CSS specs.
 </pre>
 
@@ -44,6 +45,11 @@ Property/Descriptor Values {#values}
 
 <index type=value></index>
 
+Grammar Productions / Types {#types}
+====================================
+
+<index type=type></index>
+
 Functions {#functions}
 ======================