use nav markup in userguide

Summary:
An explicit navigation markup was recenty added. Use it in
the userguide instead of ad-hoc -> or -> chains.

Test Plan: Read the docs.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10560

Conflicts:
src/docs/user/userguide/diffusion_hosting.diviner