Add install guide for users and clean up README

* Add "Using Horizon" section to the docs, which describes
  how to install and configure Horizon. It is for operators
  who install Horizon from source code.
* Clean up README.rst as much of the information is out-of-date
  and most of it is covered by the developer docs.
* Minor update of page titles.

Change-Id: I09aab0d0bc47e07c8d663e7f7b5801e456434950
Merge "Add install guide for users and clean up README"