add contributor onboarding guide for SDK, MCP, and npm SDK

[Abiorh001]

Summary

New contributors need a clearer map of the repository and contribution paths.

Problem

The repo now spans:

• Python SDK
• MCP server
• docs and examples
• planned npm SDK

Contributors need one guide that explains where to start.

Acceptance criteria

• contributor guide explains repo areas
• local setup is documented
• Redis and Circle sandbox expectations are documented
• guidance exists for picking issues and opening PRs