User Guide - fix and adjustment

Hi Rich,

This branch is meant to be merged after https://github.com/sez11a/liferay-docs/pull/946. It fixes an issue in which one leading header was an h2 instead of an h1 and it moved the introduction article content from using-liferay-portal/home.markdown to using-liferay-portal/articles/00-intro/intro.markdown. The latter change makes the introduction a peer article of all the other chapter intros as proposed in https://issues.liferay.com/browse/LRDOCS-1262.