docs: add quick reference guide and finalize contributor setup

[cheehenn]

Summary

• add QUICK_REFERENCE.md at the repo root so developers can skim the crate’s APIs, feature flags, and conversion semantics
• link the README to the quick reference and explain why the serde feature uses a tagged JSON format
• ignore local development folders (.venv/, .idea/) and remove previously committed JetBrains config
• sync CONTRIBUTING.md with the current maintainer workflow

Testing

• not run (documentation and gitignore updates only)