Provide general documentation on how to use perfomance tools

Summary: Ref T8617. Provide general documentation with tools for debugging hangs and slow pages. Update DarkConsole docs and discuss how to use Services and XHProf. Explain what Multimeter is for and how to use it. Update XHProf docs and provide some usage hints.

Test Plan: Read documentation.

Reviewers: joshuaspence, btrahan

Reviewed By: joshuaspence, btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T8617

Differential Revision: https://secure.phabricator.com/D13359