fix: document maintainer invites through private repo

[SMillerDev]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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A very minimal version of documenting how to automatically add maintainers.

[Copilot]

Pull Request Overview

This PR updates the maintainer onboarding and offboarding documentation to reference the automated user management tooling in the private repository. The changes streamline the process by directing maintainers to make pull requests to a configuration file rather than manually managing team memberships.

• Updated maintainer invitation instructions to use the user-management tooling
• Added reference to remove maintainers from the automated tooling during offboarding

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[MikeMcQuaid]

Perfect, thank you @SMillerDev!