docs: add CONTRIBUTING.md

[petercorke]

Summary

MVTB never had a CONTRIBUTING.md. Adds one, structured the same way as the rewritten RTB/bdsim versions (companion PRs), filled in with MVTB's actual setup:

• Tooling: ruff (lint/import-sort) + black (format) + pyright (type checking) -- confirmed via pyproject.toml
• Real CI test command (python -m pytest --timeout=50 --timeout-method=thread)
• Conventional Commits PR-title note (commitlint.yml already enforces it)
• Pointer to good first issue/help wanted labels

Test plan

• Verified tooling (ruff/black/pyright) via pyproject.toml
• Verified commitlint.yml exists and actually checks PR titles
• Verified good first issue/help wanted labels exist

[codacy-production]

Up to standards ✅

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