Add doc for expand

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

docs/user/ppl/cmd/expand.rst

@@ -0,0 +1,46 @@
+PPL ``expand`` command
+=======================
+
+Description
+-----------
+
+Using ``expand`` command to flatten an array of nested type.
+
+Syntax
+------
+
+``expand <field> [As alias]``
+
+- *field*: The field to be expanded (exploded). Currently only nested arrays are supported.
+- *alias*: (Optional) The name to use instead of the original field name.
+
+Usage Guidelines
+----------------
+
+The expand command produces a row for each element in the specified array or map field, where:
+
+- Array elements become individual rows.
+- When an alias is provided, the exploded values are represented under the alias instead of the original field name.
+- This can be used in combination with other commands, such as ``stats``, ``eval``, and ``parse`` to manipulate or extract data post-expansion.
+
+Examples
+--------
+
+Given a dataset ``move`` with the following data:
+```
+{"name":"abbas","age":24,"address":[{"city":"New york city","state":"NY","moveInDate":{"dateAndTime":"19840412T090742.000Z"}}]}
+{"name":"chen","age":32,"address":[{"city":"Miami","state":"Florida","moveInDate":{"dateAndTime":"19010811T040333.000Z"}},{"city":"los angeles","state":"CA","moveInDate":{"dateAndTime":"20230503T080742.000Z"}}]}
+```
+The following query expand the address field and rename it to addr:
+
+PPL query::
+
+    PPL> source=move | expand address as addr;
+    fetched rows / total rows = 3/3
+    +-------+-----+-------------------------------------------------------------------------------------------+
+    | name  | age | addr                                                                                      |
+    |-------+-----+-------------------------------------------------------------------------------------------|
+    | abbas | 24  | {"city":"New york city","state":"NY","moveInDate":{"dateAndTime":"19840412T090742.000Z"}} |
+    | chen  | 32  | {"city":"Miami","state":"Florida","moveInDate":{"dateAndTime":"19010811T040333.000Z"}}    |
+    | chen  | 32  | {"city":"los angeles","state":"CA","moveInDate":{"dateAndTime":"20230503T080742.000Z"}}   |
+    +-------+-----+-------------------------------------------------------------------------------------------+