Install guide: recommend 64-bit OS

Add some text to the install guide to recommend installing a 64-bit
operating system. Also links to Ops Guide for system requirements.

Motivated by this comment in the docs:
http://docs.openstack.org/havana/install-guide/install/apt/content/nova-boot.html#comment-1146233396

Change-Id: Iabe612024be0f6fcf335cf2daaf3ea77bed3cd86
backport: havana
Fixes the title of Doc website.

Change-Id: Ie6cefefd5e60a750ed04fa64fa7335ae1b3edc78
Merge "Install guide: recommend 64-bit OS"
Merge "Fixes the title of Doc website."
Fix reference in Security Guide

This is the first paragraph of the chapter, the reference is to the
previous one.

Change-Id: I31f4841ddbb1277b9e4161ee76b06ea4581ff2e2
Closes-Bug: #1253794
Merge "Fix reference in Security Guide"