Update README with project overview and setup instructions

[robpitcher]

The root README needs a refresh to help new contributors quickly understand and run the projects in this repo.

Scope

• Add a concise project overview (purpose, tech stack, high-level folder structure)
• Quickstart section: prerequisites, dev container notes, setup steps
• How to run: minimal commands for key subprojects (Node, .NET, Java/Quarkus/Spring, C++)
• Testing: how to run tests where available
• Contribution guidelines: link to existing policies, basic code style, and PR expectations
• Reference existing files: LICENSE, SECURITY.md, SUPPORT.md, CODE_OF_CONDUCT.md
• Links to challenge folders and example apps

Acceptance Criteria

• README includes an overview, Quickstart, run/test instructions, and contribution guidance
• All repository file links and folder references are valid
• Commands are verified to work in the dev container environment
• Clear, scannable structure with headings and short lists

Optional

• Badges (build/test status if applicable)
• Screenshot(s) or GIF(s) to preview sample apps