Add review app rule for updating handbook

When making a handbook update, please leave a review app link to the affected handbook page in the MR description

CA handbook: https://specify-review-app-link-for-handbook-updates.about.gitlab.com/handbook/marketing/developer-relations/community-advocacy

Handbook guidelines: https://specify-review-app-link-for-handbook-updates.about.gitlab.com/handbook/handbook-usage/#handbook-guidelines