lang-docs team adoptions charter follow-up

@jackh726 raised a concern that was missed in the lang-docs team adopting rustlings, rust-by-example, and the book team:

So, I think moving these groups under lang-docs is good idea in terms of organization. But, what I personally would really like to see before this happens is some kind of "charter" from lang-docs on what what is expected from subteams in terms of autonomy and such. It may seem "obvious", but especially given @Chris Krycho's comment re. the book team, I think it would be really good to formalize this.

I would like to follow up with this, which would mean:

• Revising the lang-docs charter, CC @ehuss @JohnTitor as team leads
• Adding a charter for rustlings, CC @mo8it, @shadows-withal as team leads
• Adding a charter for rust-by-example, CC @marioidival as team lead
• Adding a charter for the book, CC @carols10cents, @chriskrycho as team leads

I agree it is good to document:

• What the sub-teams are expected to do
• What responsibility is delegated to the sub-teams (particularly the expected level of autonomy).

Niko provided a potentially useful example of a charter from the proposed project-goals team: https://rust-lang.github.io/rust-project-goals/admin/team.html

Start by reviewing the existing lang-docs charter and Niko's proposed project-goals team charter example. Follow the linked discussion for the unresolved concern about subteam autonomy, then draft or revise charters for lang-docs, rustlings, rust-by-example, and the book. Done means each charter documents responsibilities, delegation, and expected autonomy.