minor: adding index and link targets

Author: Sam Kleinman

source/core/sharding-internals.txt

@@ -254,8 +254,12 @@ then add an additional field to provide additional :ref:`cardinality
 <sharding-shard-key-cardinality>` if your primary key does not have
 sufficient split-ability.
 
-Indexes
--------
+.. index:: sharding; shard key indexes
+.. _sharding-internals-shard-key-indexes:
+.. _sharding-shard-key-indexes:
+
+Shard Key Indexes
+~~~~~~~~~~~~~~~~~
 
 All sharded collections **must** have an index that starts with the
 :term:`shard key`. If you shard a collection that does not yet contain