speedup section explains the user how to sppedup image build by using
tmpfs. The correct user guide to have this section, is the user guide
about image building rather than the installation user guide.
Change-Id: I96b90bd79df53db4f926a928ae3c86b888315230
Our docs are very developer focused. Lets create a separate user guide
to help new users get started.
Change-Id: I8a03920e6d3306dd0405177875ea55ccb4b40fea