Document nginx, s3 storage in Phabricator

Summary: Add nginx documentation and s3 documentation and some other doc tweaks.

Test Plan: Generated, read documentation.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, wnemay

Maniphest Tasks: T638

Differential Revision: https://secure.phabricator.com/D1426