Refresh CONTRIBUTING.md to match current repository practices

Problem

CONTRIBUTING.md no longer reliably describes current repository development and review practices. Because it mixes enforceable requirements, historical architecture, and aspirational guidance, contributors and reviewers can treat stale statements as current policy.

Examples that need an explicit audit include:

• DCO/sign-off requirements versus the checks and rules currently enforced by GitHub.
• The documented mirrored test layout versus the current repository test organization.
• Development setup, dependency-management, lint, and test commands.
• References to legacy package paths, pipeline architecture, and processing patterns.
• The distinction between required contribution policy and optional architectural guidance.

Proposed work

• Validate each instruction against the current repository, CI workflows, and supported developer workflow.
• Remove or rewrite historical guidance that no longer applies.
• Clearly label which requirements are enforced and which are recommendations.
• Update file paths, commands, examples, and links.
• Keep the guide concise enough to remain maintainable.

Acceptance criteria

• A new contributor can set up the repository and run the documented checks successfully.
• Test-placement and code-review guidance matches current practice.
• DCO/sign-off language reflects the policy maintainers actually intend to enforce.
• All referenced paths and links are valid.
• Legacy NeMo Retriever / nv-ingest guidance is removed or clearly identified as historical context.

Start with CONTRIBUTING.md and compare every instruction against the current repository, CI workflows, and supported developer workflow. Check setup, dependency, lint, test, test-placement, review, DCO/sign-off, paths, and links, including legacy NeMo Retriever and nv-ingest guidance. Done means a concise guide whose commands work, references are valid, and enforced requirements are distinct from recommendations.