Add global triagebot configuration docs

[ehuss]

Support for global configuration was added in rust-lang/triagebot#2292.

Rendered

[rustbot]

r? @Kobzol

rustbot has assigned @Kobzol.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: triagebot
• triagebot expanded to 6 candidates
• Random selection from Kobzol, Mark-Simulacrum, Urgau, apiraino, jackh726

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.