Note in contributing guide that PRs must include the template checklist

[jason-curtis]

• Closes Contributing guide should note that PRs must include the template checklist #2706
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:num or this Pull Request with :pull:num . Includes contributor name and/or GitHub username (link with :ghuser:user ).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

While working on #2705, a coding agent created the PR using gh pr create --body "...", which replaced the default PR template and omitted the checklist. This adds a short note to the contributing guide reminding contributors to include the checklist when their workflow overrides the template.

[cwhanse]

Thanks again @jason-curtis