Add updated logo to source directory for ReadTheDocs

- Copy logo to docs/source/_static/img/ so it's available for ReadTheDocs builds
- ReadTheDocs builds from source, not from pre-built HTML files
- This ensures the custom logo appears on the hosted documentation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jeremymanning