docs: add LangChain integration page to MCP Toolkit section

Created by: claudiovb

Description

Add a dedicated documentation page for the LangChain + MCP integration introduced in tetherto/wdk-mcp-toolkit#10.

What's included

New page: ai/mcp-toolkit/langchain.md

• Documents the serve CLI command and its built-in chain/protocol registry
• Quick start snippets for Python and TypeScript using MultiServerMCPClient
• Configuration reference: environment variables, WDK_CHAINS, custom wdk.config.json
• LLM provider support table (OpenAI / Anthropic)

Navigation updates

• Added entry in SUMMARY.md
• Added navigation card in MCP Toolkit landing page
• Added cross-link hint in Get Started page

Type of change

• Documentation update