Improve pygments.enabled discussion

Summary: Ref T6533. Provide better instructions around installing, enabling, and troubleshooting Pygments.

Test Plan: Read documentation, viewed config option, clicked links.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, fabe

Maniphest Tasks: T6533

Differential Revision: https://secure.phabricator.com/D11053