Create a Riskified-specific postmortem checklist

[KirilYurovnikRiski]

Replace the original postmortem checklist from the PagerDuty docs with a Riskified-specific version.

https://github.com/KirilYurovnikRiski/postmortem-docs/blob/master/docs/index.md?plain=1#L33 https://github.com/KirilYurovnikRiski/postmortem-docs/blob/master/mkdocs.yml#L36