Update ppl documentation index for new functions (opensearch-project#3868)

Signed-off-by: Shenoy Pratik <sgguruda@amazon.com>

Author: ps48

docs/user/ppl/index.rst

@@ -114,6 +114,12 @@ The query start with search command and then flowing a set of command delimited
 
   - `IP Address Functions <functions/ip.rst>`_
 
+  - `Collection Functions <functions/collection.rst>`_
+
+  - `Cryptographic Functions <functions/cryptographic.rst>`_
+
+  - `JSON Functions <functions/json.rst>`_
+
 * **Optimization**
 
   - `Optimization <../../user/optimization/optimization.rst>`_