Early branch creation: Update monthly documentation on the handbook

Context

gitlab-com/gl-infra/software-delivery&1 (closed) will move the active stable branch generation to be done one week earlier in the release schedule to allow for thorough testing and timely backports. This will increase the quality, safety, and reliability of the monthly release while enabling a better developer experience and confidence.

Update monthly documentation on the handbook

The monthly release documentation should be updated to reflect the new process

https://handbook.gitlab.com/handbook/engineering/releases/monthly-releases/