chg: [data-structure] make it more generic

backend/data-structure.md

@@ -3,6 +3,19 @@
 The data structure is based on a Redis-compatible data store. [kvrocks](https://github.com/bitleak/kvrocks) is the Redis-compatible data store used for CyCAT
 but any compatible Redis data store can be used.
 
+# UUID k/v
+
+Each UUID inserted in CyCAT has at least an entry in the backend with the following format:
+
+- `u:<UUID>` -> `value type`
+
+## Value type available
+
+|value|type|
+|-----|-----------------------------------------|
+|`1   | Publisher                               |
+| 2   | Project                                 |
+
 # Statistics
 
 ## Automatic API statistics