Added an article about setting up basic auth for previewing.
Added a section to preview FAQ on how to use preview behind a firewall.
Removed outdated article about web sockets.

Also, moved the new web sockets article to Preview category.
Updated the side bar to link to basic auth for preview article and
updated websockets article.
Merge pull request #6 from action-io/preview-articles-update

Update articles about Previewing feature