Docs: add testing and CI documentation for Odoo addons

[laura30llorente]

Summary

This PR introduces a complete guide for our testing strategy, quality assurance stack and CI workflow tailored specifically for our Odoo custom addons.

It is designed to help current and future developers understand how to write, run and maintain tests within the project without needing a live Odoo database.

Changes included

• Testing stack: Documented the use of pytest and its ecosystem (asyncio, mock, cov, dotenv, testdox), along with Ruff and Safety, including direct links to their official PyPI/Documentation pages.
• Project structure & Config: Explained the tests/ directory layout organized by Odoo module, pytest.ini setup and the critical conftest.py containing the custom Odoo environment mocking strategy.
• Local execution: Added clear instructions for running tests via the Makefile (make test, make test-cov) from the repository root.
• CI pipeline: Documented the GitHub Actions workflow for automated testing in the cloud.
• Developer guide: Added a practical, step-by-step tutorial on how to test Odoo modules safely using in-memory classes like FakeRecord and FakeModel to simulate the Active Record pattern.

Closes #5