DOCS-13883 Update left nav index

Author: mungitoperrito

source/reference/operator/aggregation.txt

@@ -1065,6 +1065,7 @@ Alphabetical Listing of Expression Operators
    /reference/operator/aggregation/pow
    /reference/operator/aggregation/push
    /reference/operator/aggregation/radiansToDegrees
+   /reference/operator/aggregation/rand
    /reference/operator/aggregation/range
    /reference/operator/aggregation/reduce
    /reference/operator/aggregation/regexFind