Link to Diffusion for Ubuntu/Red Hat install scripts

Summary: Fixes T8751, Swaps out links to Diffusion. Also upped font size on Document monospace.

Test Plan: Rebuild Diviner docs, click on new links.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8751

Differential Revision: https://secure.phabricator.com/D13566