Expand and organize triagebot documentation.

[ehuss]

This expands the triagebot documentation to cover everything in a consistent style. There are multiple goals here:

• Remove the wiki documentation so that there is only one place the documentation lives.
• Have a consistent style across the different features.
• Make the documentation user-centric where the instructions on how to use something are up-front (instead of focusing on how to configure something).
• Give each feature a separate page, since very large pages on mdbook aren't great for navigation.
• Add missing features, and to update any missing information from existing ones.
• Provide a quick summary cheat-sheet.

Closes https://github.com/rust-lang/triagebot/issues/1697