WebMCP guides

This first batch publishes 2 of the 3 upstream WebMCP guides; the rest are converted and will be released in later batches.

• Agentic Forms — Expose client-side functionality as tools to AI agents by annotating standard HTML forms with WebMCP attributes.
• Agentic JavaScript Tools — Programmatically register client-side JavaScript functions as tools for AI agents using the WebMCP Imperative API.