Add Software Supply Chain Security Tier 2 On-Call handbook pages

Starting documentation and guidance for SSCS tier 2 on call that uses https://handbook.gitlab.com/handbook/engineering/devops/oncall/ as a starting point. Credits to @dsatcher for setting that up 🙇

As we review, and get questions, we can adapt these to any SSCS specific processes.

Relates to https://gitlab.com/gitlab-com/software_supply_chain_security-sub-department/-/issues/432