Expand README with project overview and status badges

[toddysm]

Rewrites the repository README to better explain its purpose and direction.

• Describes the repo as a demonstration of the CSSC framework and the Custos project.
• Documents the three current sample apps (Python, Node.js, Java) and supporting supply chain capabilities.
• Adds a 'What's coming next' section noting future AI-related artifacts (models, datasets) and how they will be secured.
• Adds a repository layout table and links to the detailed docs under docs/.
• Adds issue/PR/license status badges at the top.

[Copilot]

Pull request overview

This PR rewrites the repository README to provide a clearer project description, document current sample apps, and add navigation/metadata (badges, repo layout, doc links) so new contributors can quickly understand what the repo contains and where to find details.

Changes:

• Adds GitHub issues/PR/license badges and expands the intro/overview content.
• Documents the three sample applications and existing supply-chain-related workflows/conventions.
• Adds sections for “What’s coming next”, repo layout, and pointers into docs/.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.