docs: add status state diagram (#742)

.docs/content/1.concepts/2.tasks.md

@@ -28,3 +28,33 @@ graph TD
   - creation of Task Duplicate (with link for monitoring and number of retry) and transfer output creation responsibility
 - Task resubmission
   - copy of task metadata with new id
+
+## Status state diagram
+
+```mermaid
+stateDiagram-v2
+    [*] --> Creating: Task is created in the db
+    Creating --> Pending: Dependencies are not available<br/>Creation rpc succeeds
+    Pending --> Submitted: Dependencies are available<br/>Task is inserted into the queue<br/>Session is running
+    Submitted --> Paused: Session is paused
+    Paused --> Submitted: Session is resumed
+    Pending --> Paused: Dependencies are available<br/>Session is paused
+    Submitted --> Dispatched: Task is acquired
+    Dispatched --> Processing: Task is sent to the worker
+    Dispatched --> Submitted: Task acquisition timeout
+    Processing --> Completed: Task succeeds
+    Processing --> Error: Task fails, no retry
+    Processing --> Retried: Task fails, recovery possible<br/>A new task is created
+    Processing --> Timeout: Task exceeds MaxDuration
+    Creating --> Cancelled: Task is marked as cancelled
+    Submitted --> Cancelled: Task is marked as cancelled
+    Dispatched --> Cancelled: Task is marked as cancelled
+    Processing --> Cancelled: Task is hardly cancelled
+    Paused --> Cancelled: Task is marked as cancelled
+    Dispatched --> Paused: Session is paused
+    Cancelled --> [*]
+    Timeout --> [*]
+    Completed --> [*]
+    Retried --> [*]
+    Error --> [*]
+```