Add missing documentation link to index page.

Author: simonwo

lib/importer/assets/docs/index.html

@@ -140,6 +140,7 @@ <h2 class="govuk-heading-s">Introduction</h2>
                   <li><a class="govuk-link" href="./what-it-is.html">What it is</a></li>
                   <li><a class="govuk-link" href="./how-it-works.html">How it works</a></li>
                   <li><a class="govuk-link" href="./start.html">Get started</a></li>
+                  <li><a class="govuk-link" href="./configuration.html">Configuration</a></li>
                   <li><a class="govuk-link" href="./articles.html">Articles</a></li>
                 </ul>