Add alert definition section to alert playbook template

All alerts have an alert definition. Currently, it is difficult to find some of them and would be much easier to just have them linked in the playbook.

Also, sometimes we don't know whether it's ok to adjust an alert and we have to guess as to how much to adjust it. It would be good to also have a standardized section in the alert playbook template for some additional context around this.

Related to https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/issues/25384