DOCS-309 adding a stable link

Author: Sam Kleinman

source/administration/sharding-architectures.txt

@@ -111,6 +111,8 @@ created subsequently, may reside on any shard in the cluster.
    databases and sharding, has nothing to do with the term
    :term:`primary` in the context of :term:`replica sets <replica set>`.
 
+.. _sharding-high-availability:
+
 High Availability and MongoDB
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