Only include content pages in ordered collection

[paulrobertlloyd]

With the addition of a settings.scss file, we can see that this is being included in the sub-navigation, albeit hidden because it doesn’t have a title:

<ul class="x-govuk-sub-navigation__section">
  <li class="x-govuk-sub-navigation__section-item">
    <a class="x-govuk-sub-navigation__link" href="/govuk-eleventy-plugin/assets/sass/settings.css"></a>
  </li>
  <li class="x-govuk-sub-navigation__section-item">
    <a class="x-govuk-sub-navigation__link" href="/govuk-eleventy-plugin/get-started/">Get started</a>
  </li>
  …

This PR changes the ‘ordered’ collection that populates the side navigation items to only search for Markdown files.