DOC: Add single PDF cover page source

This file is exported to PDF then concatenated with the Book 1 and Book
2 PDF to create a single PDF document. It provides an overview, a
notice that the PDF contains two books that are concatenated, and a
pointer to the location of the second book. The single PDF is placed at
https://itk.org/ItkSoftwareGuide.pdf , which is the URL used e.g. by
Luis Ibanez for many years on the mailing lists.

Co-authored-by: Hans Johnson <[email protected]>

Author: thewtex