Don't add links for directories in guides structure

[arthurevans]

To match our current outline, I'd like to omit link for the directories themselves.

I did this by replacing the index.md in a given directory with a stub that contains only metadata, and rewriting the nav template like this:

      <side-nav>
        {%- set navSections = collections.guide | eleventyNavigation -%}
        {# Uncomment to use prebuilt nav
        {{ navSections | eleventyNavigationToHtml | safe }}
        #}
        <ul>
        {%- for entry in navSections %}
          <li{% if entry.url == page.url %} class="my-active-class"{% endif %}>
            {%- if entry.children | length %}
              <span class="sectionHead">{{ entry.title }}</span>
              {% set navPages = entry.children %}
              <ul>
              {%- for entry in navPages %}
                <li{% if entry.url == page.url %} class="my-active-class"{% endif %}>
                  <a href="{{ entry.url | url }}">{{ entry.title }}</a>
                </li>
              {%- endfor %}
              </ul>  
            {%- else %}
              <a href="{{ entry.url | url }}">{{ entry.title }}</a>
            {%- endif %}
          </li>
        {%- endfor %}
        </ul>
      </side-nav>

Not sure if there's a simpler way to do this.