ci: add a direct link to guide in buildchecker msg
Created by: jhchabran
Being on support duty and seeing some buildkite-main failures made me rethink about our slack message from Build checker. We are mentioning the CI playbook, but it's a bit daunting and does not tell a possible newcomer that the playbook contains a step by step guide to help themself.
The emoji and the direct link could improve this.
Test plan
N/A, purely textual change.