Fix website relative navigation from first page to index

[swquinn]

The index.html is generated from the SUMMARY.md file, but in relative navigation, i.e. "previous chapter", the navigation link tries to take you to README.html.

Here is the relevant excerpt of rendered HTML from the first chapter of an example book:

<article class="container">
  <div class="columns">
    <div class="column col-1 col-mr-auto">
      <div class="p-fixed" style="top: 50vh">

        <!-- Right here! Here's the culprit! -->        
        <a href="README.html">
          <i class="icon icon-arrow-left"></i>
        </a>

      </div>
      <div class="column col-8 col-mx-auto">
        <h1>Chapter 1</h1>
        
        <p>Etiam sit amet augue ...</p>

        <!-- ... more -->

      </div>
    </div>
  </div>
</article>

[swquinn]

The fix in commit hash dcd0b86 is a temporary fix. The biggest problem is that it hard codes the solution. I'll need to revisit this to create a more workable solution in the long run.