Add documents for UUID automatic generation. (#215)

Author: badmonster0

docs/docs/core/flow_def.mdx

@@ -167,8 +167,22 @@ If the data slice has `Struct` type, you can obtain a data slice on a specific s
 
 ## Data Collector
 
-A **data collector** can be added from a specific data scope, and it collects multiple entries of data, represented by data slices in the same or children scope.
+A **data collector** can be added from a specific data scope, and it collects multiple entries of data from the same or children scope.
+
+### Collect
+
 Call its `collect()` method to collect a specific entry, which can have multiple fields.
+Each field has a name as specified by the argument name, and a value in one of the following representations:
+
+*   A `DataSlice`.
+*   An enum `cocoindex.GeneratedField.UUID` indicating its value is an UUID automatically generated by the engine.
+    The uuid will remain stable when other collected input values are unchanged.
+
+    :::note
+
+    An automatically generated UUID field is allowed to appear at most once.
+
+    :::
 
 For example,
 
@@ -182,17 +196,18 @@ def demo_flow(flow_builder: cocoindex.FlowBuilder, data_scope: cocoindex.DataSco
   demo_collector = data_scope.add_collector()
   with data_scope["documents"].row() as document:
     ...
-    demo_collector.collect(filename=document["filename"], summary=document["summary"])
-
+    demo_collector.collect(id=cocoindex.GeneratedField.UUID,
+                           filename=document["filename"],
+                           summary=document["summary"])
   ...
 ```
 
 </TabItem>
 </Tabs>
 
-Here the collector is in the top-level data scope, and it collects `filename` and `summary` fields from each row of `documents` field.
-
-To specify what to do with the collected data, call a specific method on the collector.
+Here the collector is in the top-level data scope.
+It collects `filename` and `summary` fields from each row of `documents`,
+and generates a `id` field with UUID and remains stable when `filename` and `summary` are unchanged.
 
 ### Export