docs: add README, CONTRIBUTING guide, and project icon

[Amnoor]

Summary

This PR introduces the complete public-facing documentation for the Runtime Node project. It adds the main README.md covering what the image is, why it exists, a feature comparison table against node:latest and node:alpine, quick start instructions, usage guidance with a sample multi-stage Dockerfile, registry and platform information, CI/CD quality guarantees, versioning policy, and an acknowledgements section. It also adds CONTRIBUTING.md covering the full contributor workflow — branching strategy, branch naming conventions, Conventional Commits usage, PR structure requirements, the CI pipeline, the distroless guarantee, the release process, and Dockerfile constraints.

Additionally, the project's SVG icon is introduced under .github/assets/ and is referenced in both documents.

Files Changed

Added:

• .github/
  • assets/
    • icon.svg
• CONTRIBUTING.md
• README.md

Modified:

• None

Deleted:

• None

Key Changes

• Added README.md with a full project overview, a "What Is Runtime Node?" and "Why Runtime Node?" narrative, a key features table, a side-by-side comparison table against node:latest and node:alpine across size, shell, package manager, vulnerabilities, attack surface, NODE_ENV preset, distroless status, multi-arch support, and provenance/SBOM.
• Added quick start pull and node --version smoke test commands for both Docker Hub and GHCR registries.
• Added a usage section with a complete example multi-stage Dockerfile showing how to use Runtime Node as a base image, including a note on exec-form ENTRYPOINT requirements due to the absence of a shell.
• Added versioning documentation covering the SemVer-NodeVersion tag format and immutability guarantee.
• Added an acknowledgements section crediting Node.js, Hadolint, and all Docker GitHub Actions used in the CI/CD pipeline, plus a contributors table.
• Added CONTRIBUTING.md with a full contributor guide covering: branch strategy (main, develop, <type>/<short-description>), branch naming conventions, Conventional Commits format and allowed types with examples, PR title and description structure requirements, CI pipeline documentation (Hadolint lint job + multi-platform build and test matrix job with all four integrity tests), the distroless guarantee manifest listing every permitted image component and its path and purpose, the release process triggered by v* tags, and Dockerfile constraints that must be respected at all times.
• Added .github/assets/icon.svg — the project's SVG icon featuring a metallic hexagon and radial green energy gradient, referenced in both README.md and CONTRIBUTING.md.