Surface agent SDK examples from toolhive repo in the docs

[jerm-dro]

User Story

As a ToolHive user attempting to build my own agent service, I can't easily discover
the agent SDK examples that show how to use ToolHive. Without documentation pointing
to the examples, they're invisible even if they exist in the repo.

Context

The toolhive repo contains worked examples for building agents on ToolHive using
popular SDKs (LangChain, LangGraph, Google ADK) under examples/agents/. These need
to be surfaced in the docs-website so users can find and follow them.

Requirements

1. Add a new guide or section (e.g., under "Guides" or a new "Building Agents" section)
   that introduces the agent SDK examples
2. Link to the examples/agents/ directory in the toolhive repo for each SDK
3. Include a brief description of each example and what the user will build

Acceptance Criteria

• Docs include a discoverable entry point for agent SDK examples
• Each of the three SDKs (LangChain, LangGraph, Google ADK) is linked and described
• Links point to the correct directories in the toolhive repo
• Content is reviewed for accuracy against the actual examples