Pre-RFC: Mentoring and Onboarding

[jamesmunns]

In order to reduce bus factor, and to ensure we have a reasonable amount of active members on each of our teams, we would like to come up with a plan for onboarding contributors to the WG, and have a plan for mentoring them.

This will allow contributors to have a solid path on how they can get started, and what kind of support they can expect, as well as give "code maintainers" guidance on how they can help people get started.

[jamesmunns]

Closed by #378