docs: update user guide to include examples for date_part and extract functions

Author: kosiew

docs/source/user-guide/common-operations/functions.rst

@@ -78,6 +78,14 @@ Convert to timestamps using :py:func:`~datafusion.functions.to_timestamp`
 
     df.select(f.to_timestamp(col('"Total"')).alias("timestamp"))
 
+Extracting parts of a date using :py:func:`~datafusion.functions.date_part` (alias :py:func:`~datafusion.functions.extract`)
+
+.. ipython:: python
+
+    df.select(
+        f.date_part(literal("month"), col('"event_time"')).alias("month"),
+        f.extract(literal("day"), col('"event_time"')).alias("day")
+    )
 String
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