Docs: Expand CONTRIBUTING.md and update Brev Launchable architecture docs.

[brycelelbach]

Summary

• Expand CONTRIBUTING.md with sections for developer setup, branches, commit message format, pre-commit hooks, development/validation/utility scripts, commit signing, and fork PR guidelines.
• Add brev/requirements-dev.txt with the minimal Python developer dependencies (pre-commit, nbformat, nbconvert, hpccm).
• Update docs/brev_launchable_architecture.md with tutorial directory structure, notebook naming conventions, exercise vs. solution notebook semantics, test infrastructure, and correct Docker Compose service names (nsys/ncu replacing the old nsight).

[github-actions]

❌ Link Check Failed

Broken links were detected in this PR.

Please check the workflow run logs for details on which links are broken.

Common fixes:

1. Typo in URL - Check for spelling mistakes in the link
2. Outdated link - The page may have moved or been deleted
3. Relative path issue - Ensure relative links use the correct path
4. External site down - If the external site is temporarily down, you can add it to brev/.lycheeignore

To test links locally:

./brev/test-links.bash .

📚 Lychee documentation