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