[Docs] Update and expand http://gohugo.io/overview/usage/

[Docs] Update and expand gohugo.io/overview/usage/

The `hugo help` output as shown in http://gohugo.io/overview/usage/
was not yet updated for v0.13. Thanks to @alebaffa for the heads up!

Also:
 - Clarify that, after using `hugo server`, the bare `hugo` command
   need to be run before deployment.
 - Add a section on running `hugo` as production web server,
   and add links to two blog posts of two Hugo users sharing
   their experience.

Partially fixes: #852 and #937