docs: fix diagram how-to version markers and remove broken mermaid

- Clarify that visual clustering is new in 2.1, not multi-schema support
- Remove mermaid rendered example (mkdocs-jupyter doesn't process it)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/how-to/read-diagrams.ipynb

@@ -1679,17 +1679,11 @@
     "```"
    ]
   },
-  {
-   "cell_type": "markdown",
-   "id": "kpn3hjsinh",
-   "source": "**Rendered example:**\n\n```mermaid\nflowchart LR\n    classDef manual fill:#90EE90,stroke:#006400\n    classDef lookup fill:#D3D3D3,stroke:#696969\n    classDef computed fill:#FFB6C1,stroke:#8B0000\n    classDef imported fill:#ADD8E6,stroke:#00008B\n    classDef part fill:#FFFFFF,stroke:#000000\n\n    subgraph __main__\n        Session[Session]:::manual\n        SessionQC([SessionQC]):::computed\n        Subject[Subject]:::manual\n    end\n\n    Session --> SessionQC\n    Subject --> Session\n```",
-   "metadata": {}
-  },
   {
    "cell_type": "markdown",
    "id": "pqet0vo8pwp",
    "metadata": {},
-   "source": "## Multi-Schema Pipelines\n\n*New in DataJoint 2.1*\n\nReal-world pipelines often span multiple schemas (modules). Tables are automatically grouped by schema, with the Python module name shown as the group label."
+   "source": "## Multi-Schema Pipelines\n\nReal-world pipelines often span multiple schemas (modules). \n\n*New in DataJoint 2.1:* Tables are automatically grouped into visual clusters by schema, with the Python module name shown as the group label."
   },
   {
    "cell_type": "code",