Add source links and a little documentation about Diviner

Summary: Ref T988. Links up the "Declared:" property to point at a repository browser, if one exists.

Test Plan: Viewed a class document, saw a link, clicked it, got the definition.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6820