Add context to readme from WCUS Contributor Day

[jeffpaul]

This pull request updates the README.md to provide clearer, more detailed information about the AI Experiments Plugin. It expands on the purpose, design goals, and planned features, and improves instructions for community involvement with updated links and descriptions.

Documentation improvements:

• Expanded the "Purpose" section to include direct references and links to the PHP AI Client SDK, Abilities API, and MCP Adapter, clarifying how the plugin integrates all AI Building Blocks.
• Added context about the plugin being a Canonical Plugin and its alignment with the Features as Plugins model, to help users understand its path to WordPress core.
• Enhanced the "Design Goals" section with more detail on integration, user experience, and control, specifically mentioning Gutenberg and admin flows.
• Expanded the "Planned Features" section with specific feature links (e.g., Title Generation, Excerpt Generation, Image Generation) and added new features such as Contextual Tagging and Alt Text Generation.

Community engagement:

• Improved the "How to Get Involved" section with direct links to the WordPress Slack channel, GitHub issues for feedback, and release/testing instructions.