Update the link for Deployment Blockers in Release Manager index.md

The link in the Deployment Blocker section of the documentation seem to be outdated (https://gitlab.com/gitlab-org/release/docs/-/blob/master/release_manager/index.md#deployment-blockers). I have update it to point to https://handbook.gitlab.com/handbook/engineering/deployments-and-releases/deployments/#deployment-blockers instead of https://handbook.gitlab.com/handbook/engineering/releases/#deployment-blockers.

This is to make sure that users/readers will able to find the process/steps that we are referring to on our docu.