temporarily replace mermaid with PNG

docs/en/development/architecture.md

@@ -198,19 +198,7 @@ Each slot can be seen as an independent state machine with the following states:
 
 Note that `allocated` slot can be in two different states: `granted` and `acquired`. The former is a transitional state, that actually should be short (from the instant when a slot is allocated to a query till the moment when the up-scaling procedure is run by any thread of that query).
 
-```mermaid
-stateDiagram-v2
-    direction LR
-    [*] --> free
-    free --> allocated: allocate
-    state allocated {
-        direction LR
-        [*] --> granted
-        granted --> acquired: acquire
-        acquired --> [*]
-    }
-    allocated --> free: release
-```
+![state diagram](@site/docs/en/development/images/concurrency.png)
 
 API of `ConcurrencyControl` consists of the following functions:
 1. Create a resource allocation for a query: `auto slots = ConcurrencyControl::instance().allocate(1, max_threads);`. It will allocate at least 1 and at most `max_threads` slots. Note that the first slot is granted immediately, but the remaining slots may be granted later. Thus limit is soft, because every query will obtain at least one thread.