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

[ncarenton]

Summary

Port of the contributing documentation improvement from OpenCTI connectors (OpenCTI-Platform/connectors@b3b61c0).

The CI already validates PR titles via validate-pr-title.yml, but contributors weren't given a clear reminder of the expected format. This PR surfaces it upfront.

Changes

• .github/PULL_REQUEST_TEMPLATE.md: add PR title format block inside the opening comment so it's visible before the contributor clears the template
• CONTRIBUTING.md: expand the "Titles" bullet in the conventions section with a code block, field-by-field breakdown, and examples

Closes #286

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.19%. Comparing base (573051a) to head (5d074fc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #287   +/-   ##
=======================================
  Coverage   73.19%   73.19%           
=======================================
  Files          53       53           
  Lines        2350     2350           
=======================================
  Hits         1720     1720           
  Misses        630      630           

Flag	Coverage Δ
connectors	73.19% <ø> (ø)

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[Copilot]

Pull request overview

This PR improves contributor-facing documentation to make the repository’s Conventional Commit-style PR title format (including required issue reference) more visible at PR creation time and clearer in contributing guidelines.

Changes:

• Adds a PR title format reminder to the top comment of .github/PULL_REQUEST_TEMPLATE.md
• Expands the “Titles” convention in CONTRIBUTING.md with a format block, field breakdown, and examples

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
CONTRIBUTING.md	Expands the titles convention with a dedicated format block and examples.
.github/PULL_REQUEST_TEMPLATE.md	Surfaces the expected PR title format at the top of the PR template for contributors.

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