Add setup guides for GitHub OAuth and local CLI usage (#415)

- Introduced a comprehensive guide for setting up GitHub OAuth authentication, detailing the flow, architecture, and implementation steps.
- Created a new setup guide for using the Copilot SDK with a locally signed-in CLI, emphasizing ease of use for personal projects and development.
- Added a scaling and multi-tenancy guide to help developers design deployments that support multiple users and concurrent sessions.
- Included configuration options, session management strategies, and production best practices across the new guides.

Author: patniko