docs: add optional bake

Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>

Author: gbartolini

CONTRIBUTING_NEW_EXTENSION.md

@@ -24,7 +24,7 @@ cd postgres-extensions-containers
 ```sh
 task prereqs      # Check if Go, Task, and Docker are ready
 task checks:all   # Validate current configurations
-task bake:all     # Build all existing extensions to confirm the Dagger engine
+task bake:all     # Optional: build all existing extensions to confirm the Dagger engine
 ```
 
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