Create a test page for PDF-based VS books, using content adapted from
https://bookshelf.vitalsource.com/reader/books/9781938168239/, an
OpenStax textboot that is available for free [1] in VitalSource.

 - Rename vitalsource.mustache => vitalsource-epub.mustache to make the
   distinction with the PDF example clearer.

 - Create vitalsource-pdf.mustache which serves as the "container" page
   for a PDF-based VS book. This is a copy of vitalsource-epub.mustache
   with the Prev/Next buttons removed and slight terminology changes
   ("chapter" => "page") to reflect how PDFs are handled in VS

 - Revise the descriptive text at the top of the VS PDF and EPUB
   examples to explain the purpose more clearly and relate them to how
   books are presented in the Bookshelf store, where the terms
   "reflowable" and "fixed" are used instead of "EPUB" and "PDF".

 - Add a very lightly edited copy of a real content page from a VS book
   in vitalsource-pdf-page.mustache. This is the HTML content from the
   iframe displaying a page in a PDF book in VS. The only changes I have
   made are small block of CSS customizations at the top (see comment)
   and a change to the URL of the rendered PDF page.

 - Update the links to the VitalSource test pages on the dev server.

[1] Even though the book is free, you have to add it to your VitalSource
    book library before you can access it. To do that, go to
    https://bookshelf.vitalsource.com and search for the book ID (9781938168239
    in this case), then click "Open Book".