docs: add CONTRIBUTING guide

[hasansezertasan]

Context

The repo has no CONTRIBUTING file. Contributors need setup, workflow, and convention guidance. PR #16 introduced a CONTRIBUTING.rst we can adapt.

Proposed tasks

• Add CONTRIBUTING file covering: uv setup, running tests, pre-commit, conventional commits/branches/PR titles, docs build
• Link it from README.md and the issue/PR templates
• Reference the org-wide Code of Conduct

Open question: .md vs .rst. The rest of the docs are Sphinx/.rst, but .md renders natively on GitHub.