Fix: Reduce multiple H1s to H2s in Distributed Agent Runtime page (#6412)

## Why are these changes needed?

This PR fixes the issue of multiple `<h1>` headers in the Distributed
Agent Runtime documentation page. The page has more than one `<h1>`
which violates semantic HTML structure. This fix downgrades the inner
section headings (e.g., "Cross-Language Runtimes", "Next Steps") to
`<h2>`.

## Related issue number

Related issue: [#6090](#6090)

## Checks

- [x] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [x] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [x] I've made sure all auto checks have passed.

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

Author: LuluZhuu

python/packages/autogen-core/docs/src/user-guide/core-user-guide/framework/distributed-agent-runtime.ipynb

@@ -189,16 +189,21 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Cross-Language Runtimes\n",
+    "## Cross-Language Runtimes\n",
     "The process described above is largely the same, however all message types MUST use shared protobuf schemas for all cross-agent message types.\n",
     "\n",
-    "# Next Steps\n",
+    "## Next Steps\n",
     "To see complete examples of using distributed runtime, please take a look at the following samples:\n",
     "\n",
     "- [Distributed Workers](https://github.com/microsoft/autogen/tree/main/python/samples/core_grpc_worker_runtime)  \n",
     "- [Distributed Semantic Router](https://github.com/microsoft/autogen/tree/main/python/samples/core_semantic_router)  \n",
     "- [Distributed Group Chat](https://github.com/microsoft/autogen/tree/main/python/samples/core_distributed-group-chat)  \n"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": []
   }
  ],
  "metadata": {