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/supplementary/stable/agents-api.md

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+## Responses API
+
+The Responses API provides OpenAI-compatible functionality with enhanced capabilities for dynamic, stateful interactions.
+
+> **✅ STABLE**: This API is production-ready with backward compatibility guarantees. Recommended for production applications.
+
+### ✅ Supported Tools
+
+The Responses API supports the following tool types:
+
+- **`web_search`**: Search the web for current information and real-time data
+- **`file_search`**: Search through uploaded files and vector stores
+  - Supports dynamic `vector_store_ids` per call
+  - Compatible with OpenAI file search patterns
+- **`function`**: Call custom functions with JSON schema validation
+- **`mcp_tool`**: Model Context Protocol integration
+
+### ✅ Supported Fields & Features
+
+**Core Capabilities:**
+- **Dynamic Configuration**: Switch models, vector stores, and tools per request without pre-configuration
+- **Conversation Branching**: Use `previous_response_id` to branch conversations and explore different paths
+- **Rich Annotations**: Automatic file citations, URL citations, and container file citations
+- **Status Tracking**: Monitor tool call execution status and handle failures gracefully
+
+### 🚧 Work in Progress
+
+- Full real-time response streaming support
+- `tool_choice` parameter
+- `max_tool_calls` parameter
+- Built-in tools (code interpreter, containers API)
+- Safety & guardrails
+- `reasoning` capabilities
+- `service_tier`
+- `logprobs`
+- `max_output_tokens`
+- `metadata` handling
+- `instructions`
+- `incomplete_details`
+- `background`