feat: add CONTRIBUTING.md with developer workflow and conventions

[DiogoRibeiro7]

Proposal

The README has a brief "Contributing" section (3 lines) but no `CONTRIBUTING.md` with detailed guidance for contributors. Create one covering:

• Branch workflow: feature branches → `develop` → `main`
• File naming conventions (link to `shared/naming_conventions.md`)
• Integrity contract (link to `shared/integrity_contract.md`)
• How to add a new week
• How to test changes locally (run_week scripts)
• How to test changes in CI
• PR template expectations
• Solutions policy (link to `docs/solutions_policy.md`)

Why

New contributors (TAs, co-instructors) need a single entry point for development conventions.

Scope / files impacted

• `CONTRIBUTING.md` (new)
• `README.md` (link to CONTRIBUTING.md)