Add contributor and community onboarding

[cmungall]

Priority: P2

Problem

The README begins developer guidance with just docs-build but does not explain how to install uv, just, or Pandoc, how to run the complete quality suite, or which documentation files are generated. The repository has no CONTRIBUTING guide, code of conduct, support guidance, issue templates, or pull request template.

New contributors cannot reliably reproduce maintainer expectations.

Acceptance criteria

• Add CONTRIBUTING documentation for environment setup and supported platforms.
• Explain just check, check-examples, docs-build, all, package builds, and focused test commands.
• Document generated-file ownership and when to run the example renderer.
• State coding, typing, test, documentation, and changelog expectations for pull requests.
• Add support/contact guidance and a code of conduct.
• Add focused bug, feature, and documentation issue templates plus a pull request checklist.
• Document maintainer/release responsibilities and how security reports differ from ordinary issues.
• Verify setup instructions in a clean CI job or dev-container equivalent.