Make new document section into its own page

Summary:
In this patch the new document section is simplified in the
starting Krita section, and the information is moved to a new page in
reference manual section. The general concepts about color is linked
where we talk about color.

Reviewers: woltherav, Krita: Manual

Reviewed By: woltherav, Krita: Manual

Tags: Krita: Manual

Differential Revision: https://phabricator.kde.org/D19720