Create offboarding.md

[tiffanychu90]

Description

Describe your changes and why you're making them. Please include the context, motivation, and relevant dependencies.

We've cycled through several offboarding of contractors and consultants and we should keep track of several updates we need to make to our repos.

Resolves #[issue]

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation

How has this been tested?

Include commands/logs/screenshots as relevant.

Post-merge follow-ups

Document any actions that must be taken post-merge to deploy or otherwise implement the changes in this PR (for example, running a full refresh of some incremental model in dbt). If these actions will take more than a few hours after the merge or if they will be completed by someone other than the PR author, please create a dedicated follow-up issue and link it here to track resolution.

• [ ] No action required
• [x] Actions required (specified below) Do a pass and remove previous contractors and consultants from admin privileges and update references in data-infra that send notifications to closed email accounts.