docs: add the changelog for 1.0.0

Adds the 1.0.0 section to the CHANGELOG, replacing Unreleased: Drupal 12 support, field labels in prose errors, and the fixes since 1.0.0-beta1, including those in the polish MR. Merge it after the polish MR. The date is 28 October; change it if the tag goes out on another day.