docs: Enhance README with detailed usage and implementation information

[trygu]

This pull request significantly expands and clarifies the README.md documentation for the LabID token exchange service. The changes provide a comprehensive overview of the service's purpose, implementation details, endpoint behavior, and configuration options, making it much easier for developers and users to understand how LabID works and how to integrate with it. Additionally, the contributing guidelines have been reorganized for clarity.

Overview and purpose:

• Added a detailed "What it does" section explaining LabID's role as a Kubernetes token-exchange service, including its validation, group mapping, JWT signing, and endpoint exposure.
• Introduced a "Background / Why it exists" section outlining the motivations for LabID, such as centralizing authentication and simplifying group-based authorization.

[johnksv]

Better documentation than what we had previously! Great