document procedural steps for addition and removal of maintainers

[lhawthorn]

Right now, if a maintainer is added to a GitHub repo or removed, buttons are pushed by one of the GH InstructLab project owners and a script can be run to update our project wide Maintainers.md file. However, this does not update settings on other project infrastructure, e.g. mailing lists (run via Google Groups).

We need to document the procedural bits for when someone is added or steps down as a maintainer. The contributor ladder is a great start but lacks these nuts and bolts details.

[github-actions]

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