docs: add metadata/uuid page

docs/content/metadata/uuid.md

@@ -0,0 +1,22 @@
+---
+navtitle: uuid
+---
+
+# Metadata "uuid"
+
+The `uuid` metadata identifies the global Nikita session. It is shared between all the child actions and contains the UUID value in the [RFC4122](https://www.ietf.org/rfc/rfc4122.txt) format.
+
+* Type: `string`
+* Read-only
+
+## Usage
+
+The metadata can be accessed from the inside of the [action handler](/current/action/handler).
+
+```js
+nikita
+.call(({metadata: {uuid}}) => {
+  // Print the value
+  console.info(uuid) // 419196e4-48af-4eb6-9dba-4e05653f8007
+})
+```