Establish basic review group for willing rustc-dev-guide members

[jieyouxu]

See #t-compiler/rustc-dev-guide > Adjust triagebot configs @ 💬.

• Setup r? rustc-dev-guide adhoc group (triagebot config). Setup basic r? rustc-dev-guide review group #2431
• Configure a triagebot PR message to ask for reviewer opt-in. Configure PR welcome message for no auto/explicit reviewer #2450

  I would be a bit worried that having automatically assigned reviewers to every PR would discourage people from just writing docs about stuff they know well and then self-merging them to get something in the dev guide

  If we could post a message like "if you want a reviewer please run @rustbot roll-reviewer" or something on PRs that get auto-opened that'd be nice