docs: improve PR title format hint in PR template and CONTRIBUTING.md #286

Context

The CI already enforces PR title conventions via .github/workflows/validate-pr-title.yml, but contributors don't get a clear reminder of the expected format when opening a PR.

This is a port of the same improvement made in the OpenCTI connectors repository: OpenCTI-Platform/connectors@b3b61c0

Changes

• .github/PULL_REQUEST_TEMPLATE.md: add an explicit PR title format block in the comment header so contributors see it before submitting
• CONTRIBUTING.md: improve the "Titles" section with a code block showing the exact format, a bullet breakdown of each field, and examples

Expected format (enforced by CI)

type(scope?)!?: description (#123)

• type: feat | fix | chore | docs | style | refactor | perf | test | build | ci | revert
• scope: optional — use the affected area or module
• description: must start with a lowercase letter
• (#123): required linked issue number at the end