docs: add AI provider integration guide and Supported AI Providers section

Add a step-by-step guide for AI infrastructure companies to implement
ai.Model and contribute a provider to go-micro. Covers the full
lifecycle: skeleton, tool call handling, tests, registration, and PR
checklist.

Add a "Supported AI Providers" section to the project README that lists
current providers (Anthropic, OpenAI) in a table and links to the
integration guide with a call-to-action for new providers and sponsors.

Streamline the "Adding a New Provider" section in ai/README.md to point
to the new guide instead of duplicating a full code listing.

Author: claude

README.md

@@ -407,7 +407,7 @@ m := ai.New("openai",
 )
 ```
 
-**Want to add your platform?** See the [AI Provider Integration Guide](internal/website/docs/guides/ai-provider-guide.md) for how to implement `ai.Model` and submit a PR. We welcome both code contributions and sponsorships from AI infrastructure companies — reach out via a [GitHub issue](https://github.com/micro/go-micro/issues).
+**Want to add your platform?** See the [AI Provider Integration Guide](internal/website/docs/guides/ai-provider-guide.md) for how to implement `ai.Model` and submit a PR. We welcome both code contributions and sponsorships from AI infrastructure companies — reach out via a [GitHub issue](https://github.com/micro/go-micro/issues) or [Discord](https://discord.gg/jwTYuUVAGh).
 
 ## Adopters