docs: add Atlas Cloud sponsorship blog post and integration guide (#2902)

Add blog/8 announcing Atlas Cloud as an official Go Micro sponsor.
Covers the sponsorship, Atlas Cloud's platform (300+ models, OpenAI
compatibility, enterprise compliance), and how the integration
works with the ai package, ai/tools, micro chat, and micro run.

Add guides/atlascloud-integration.md with full setup instructions:
quick start, configuration options, environment variables, model
selection, tool calling with services, and provider swapping.

Add Atlas Cloud and AI Provider guides to the docs sidebar
navigation. Add sponsorship link to README header.

Co-authored-by: Claude <noreply@anthropic.com>