doc: Add PagerDuty to Gitaly team member onboarding
Relates to gitlab-org/core-platform-section/discussions#178
This is part of the dev oncall pilot of Gitaly tracked at gitlab-org/core-platform-section&4 .
Relates to gitlab-org/core-platform-section/discussions#178
This is part of the dev oncall pilot of Gitaly tracked at gitlab-org/core-platform-section&4 .