ref-guide tools should fail build if any page exists with #parents != 1

BuildNavAndPDFBody.java already has to build up a completle tree of all pages in order to generate the sidebar navigation and pdf-main-body. I'd always ment to add the code to also validate that no page winds up in the doc twice (because they are declared in the page-children of multiple "parent" files) or gets left out (because someone forgot to include them in the page-children of a "parent" file)

...but never got around to it until today.