Fix inconsistent sidebar nesting #519

The sidebar nesting logic is inconsistent which can create challenges for plugin and theme authors looking to target specific navigation levels.

Here is the docsify HTML output from a three-level sidebar navigation:

<div class="sidebar-nav">
  <ul>
    <li class="active">
      <a href="#/page">Page</a>
      <ul class="app-sub-sidebar">
        <li>
          <a class="section-link" href="#/page?id=section">Section</a>
        </li>
        <li>
          <ul class="children">
            <li>
              <a class="section-link" href="#/page?id=link">Link</a>
            </li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</div>

Take notice of how the top-level item and its children are rendered:

<li class="active">
  <a href="#/page">Page</a>
  <ul class="app-sub-sidebar">
    ...
  </ul>
</li>

The child items are rendered as a ul element which is a sibling of the a element. This makes it easy to target the ul element via CSS (a + ul) or JavaScript (aNode.nextSibling).

Now take notice of how the second level links are rendered:

<li>
  <a class="section-link" href="#/page?id=section">Section</a>
</li>
<li>
  <ul class="children">
    ...
  </ul>
</li>

Here the a and ul elements are contained in separate li items. This makes it impossible to target the ul from the a element via CSS, and more expensive via JavaScript (aNode.parentNode.nextSibling.querySelector('ul.children')). This also breaks the default list styling, as the sibling li elements render as follows:

• Page
  • Section
  • 
    • Child
    • Child

What I would hope to see is the same nesting logic applied to all levels of the sidebar navigation: links that have children are rendered as an a tag immediately followed by a ul tag containing the children:

<li>
  <a class="section-link" href="#/page?id=section">Section</a>
  <ul class="children">
    ...
  </ul>
</li>

Using this nesting logic would make targeting via CSS and JS much easier, and allow the default list styling to work for an infinite number of levels:

• Level 1
  • Level 2
    • Level 3
      • Level 4
        ...

Thanks!