docs: restore Icon Usage note in architecture.md

Agent-Logs-Url: https://github.com/Azure/apiops-cli/sessions/6362b9de-dfe9-46fc-81bb-5cb92a322e8a

Co-authored-by: EMaher <9244742+EMaher@users.noreply.github.com>

Author: Copilot

docs/architecture.md

@@ -54,6 +54,11 @@ flowchart LR
 | 4 | Merge | Approved changes are merged |
 | 5 | `apiops publish` | On merge, the publish workflow runs and applies artifact files to the target API Management instance |
 
+### Icon Usage
+
+> [!NOTE]
+> Diagrams use icons from [Font Awesome Free](https://fontawesome.com/) via Mermaid's built-in `fa:` syntax.
+
 ---
 
 ## Component Architecture