docs: expand README with config sharing, MCP notes, and more examples

- Add detailed usage examples (sandbox, search grounding)
- Document ~/.gemini folder structure and GEMINI.md
- Add MCP support section with limitations and workarounds
- Clarify OAuth and Vertex AI authentication options
- Update docker-compose with host mount and Vertex AI profile
- Add shell alias convenience section
- Add built-in tools documentation

Author: bryant-ung