📝 docs(lib): add comprehensive library documentation
- introduce library's purpose, installation, and quick start guide
- describe core types (gmailclient, clientconfig, rules) with usage examples
- explain configuration options (oauth2, config file, environment variables)
- detail error handling, async runtime requirements, and gmail query syntax
- cover performance considerations (pagination, rate limits, batch operations)
- provide logging setup, security considerations, and troubleshooting tips
- include links to cli documentation, examples, api reference, and repository
7afbb1d56d