Add onboarding/offboarding documentation to the handbook

We need a getting started page for new team members joining us that provides guidance and additional context to existing onboarding issue templates.

1. Review existing onboarding/offboarding issue templates
2. Identify links to additional documentation, resources, etc. for services and technology managed by ~"team::Observability"
3. Identify links to commonly used dashboards, applications, slack channels, etc.
4. Provide guidance and resources for how/where to go for additional help or to learn more about our platform