Output DocumentPointer as a xhtml file with a link to the document

Document pointers are pointers to unmodified published documents which
are sometimes included in a book.  An example of this is when you want
to edit the order of documents in a book, you don't need to change the
documents but you need to change the book.

```
DocumentPointer('pointer@1', {
    'title': 'Pointer',
    'cnx-archive-uri': 'pointer@1',
    'url': 'http://cnx.org/contnts/pointer@1',
    })
```

generates a xhtml file that looks like this:
```
<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:epub="http://www.idpf.org/2007/ops"
      xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
      xmlns:dc="http://purl.org/dc/elements/1.1/"
      xmlns:lrmi="http://lrmi.net/the-specification"
      >
  <head itemscope="itemscope"
        itemtype="http://schema.org/Book"
        >

    <title>Pointer</title>

    <!-- These are for discoverability of accessible content. -->
    <meta itemprop="accessibilityFeature" content="MathML" />
    <meta itemprop="accessibilityFeature" content="LaTeX" />
    <meta itemprop="accessibilityFeature" content="alternativeText" />
    <meta itemprop="accessibilityFeature" content="captions" />
    <meta itemprop="accessibilityFeature" content="structuredNavigation" />

  </head>
  <body xmlns:bib="http://bibtexml.sf.net/"
        xmlns:data="http://dev.w3.org/html5/spec/#custom"
        itemscope="itemscope"
        itemtype="http://schema.org/Book"
        >
    <div data-type="metadata">
      <h1 data-type="title" itemprop="name">Pointer</h1>
      <span data-type="document" data-value="pointer" />
      <span data-type="cnx-archive-uri" data-value="pointer@1" />    </div>

    <div>
      <p>
        Click <a href="http://cnx.org/contents/pointer@1">here</a> to read Pointer.
      </p>
    </div>
  </body>
</html>
```