docs: fix broken contributor setup instructions in CONTRIBUTING.md

[tungyhardDevOps]

Closes #303

Problem

The Before Start section of CONTRIBUTING.md documents a setup flow that doesn't work against the current repo:

• bash install.sh — there is no install.sh in the repository.
• cat main.py and uvicorn app:app — there is no main.py or app.py.
• The ```sh code fences in the section are malformed.
• "For testing use tox" — tox.ini exists, but its envlist targets Python 3.6–3.9, while the project now requires Python ≥ 3.10; the actual maintained test path is pytest via the Makefile.

(As noted on the issue, this section "needs an update.")

Fix

Rewrote Before Start and Testing to match the repo's real tooling, verified against pyproject.toml, poetry.lock, and the Makefile:

• Dev setup via Poetry: pip install poetry → poetry install → poetry shell (poetry install pulls in the dev tools: pytest, black, isort, flake8, mypy).
• Testing via make test (pytest + coverage) or pytest directly.
• Linting/formatting via make lint and make format_code.
• Fixed the malformed code fences and removed references to non-existent files.

Docs-only change — no code touched.