Improve the release note guidance in the pull request template

## Enhancement

Improve the pull request template by adding a checkbox that lets PR authors indicate whether a change needs to be included in the user-facing release notes.

This change has three benefits:

1. It makes the decision simple and low-effort for PR authors: select the checkbox when a release note is needed, or leave it unselected when it is not.
2. It provides a clearer signal for both human reviewers and AI to distinguish PRs that need release notes from those that do not.
3. It recommends that PR authors write the release note themselves. If an author marks the PR as requiring a release note but leaves the release note as `None`, a bot can generate a draft later when the release notes file is prepared.

Locate the repository's pull request template and read its existing release-note guidance first. Add the requested checkbox and recommendation for authors to provide the release note, including the stated None case; done means the template clearly distinguishes PRs that need user-facing release notes from those that do not.

Written by the indexing model from the issue text.