docs: adding supplementary markdown content to API specs (#3632)

# What does this PR do?

Adds supplementary static content to root API spec pages. This is useful for giving context behind a specific API group, adding information on supported features or work in progress, etc.

This PR introduces supplementary information for Agents (experimental, deprecated) and Responses (stable) APIs.

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## Test Plan

Documentation server renders rich static content for the Agents API group:

![image.png](https://app.graphite.dev/user-attachments/assets/fc521619-0320-4a22-9409-8ee3fb57ed0e.png)

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docs/static/experimental-llama-stack-spec.html

@@ -6479,27 +6479,33 @@
     "tags": [
         {
             "name": "Agents",
-            "description": "APIs for creating and interacting with agentic systems.",
+            "description": "APIs for creating and interacting with agentic systems.\n\n## Agents API (Experimental)\n\n> **🧪 EXPERIMENTAL**: This API is in preview and may change based on user feedback. Great for exploring new capabilities and providing feedback to influence the final design.\n\nMain functionalities provided by this API:\n\n- Create agents with specific instructions and ability to use tools.\n- Interactions with agents are grouped into sessions (\"threads\"), and each interaction is called a \"turn\".\n- Agents can be provided with various tools (see the ToolGroups and ToolRuntime APIs for more details).\n- Agents can be provided with various shields (see the Safety API for more details).\n- Agents can also use Memory to retrieve information from knowledge bases. See the RAG Tool and Vector IO APIs for more details.\n\n### 🧪 Feedback Welcome\n\nThis API is actively being developed. We welcome feedback on:\n- API design and usability\n- Performance characteristics\n- Missing features or capabilities\n- Integration patterns\n\n**Provide Feedback**: [GitHub Discussions](https://github.com/llamastack/llama-stack/discussions) or [GitHub Issues](https://github.com/llamastack/llama-stack/issues)",
             "x-displayName": "Agents"
         },
         {
-            "name": "Benchmarks"
+            "name": "Benchmarks",
+            "description": ""
         },
         {
-            "name": "DatasetIO"
+            "name": "DatasetIO",
+            "description": ""
         },
         {
-            "name": "Datasets"
+            "name": "Datasets",
+            "description": ""
         },
         {
             "name": "Eval",
+            "description": "",
             "x-displayName": "Llama Stack Evaluation API for running evaluations on model and agent candidates."
         },
         {
-            "name": "PostTraining (Coming Soon)"
+            "name": "PostTraining (Coming Soon)",
+            "description": ""
         },
         {
-            "name": "Telemetry"
+            "name": "Telemetry",
+            "description": ""
         }
     ],
     "x-tagGroups": [