Add documentation for what it means to be part of the Documentation Team

[frewsxcv]

it looks like the community team has a couple documents indicating what it means to be part of the team. we should have something similar for the docs team.

• https://github.com/rust-community/team/blob/master/rfcs/team-membership.md
• https://github.com/rust-community/team/blob/master/rfcs/observer-policy.md

I originally asked about this in a meeting agenda and opened this dedicated tracking issue