Nav tree
Nav with tree structure.
<!-- Source -->
<ul class="nav nav-tree nav-x-0 nav-y-sm flex-column">
<li class="nav-item"><a class="nav-link" href="#">Item 1</a></li>
<li class="nav-item show"><a class="nav-link" href="#">Item 2</a>
<ul class="nav nav-tree nav-x-0 nav-y-sm font-size-2 pl-3 flex-column">
<li class="nav-item"><a class="nav-link" href="#">Sub-item 1</a></li>
<li class="nav-item show"><a class="nav-link active" href="#">Sub-item 2</a>
<ul class="nav nav-tree nav-x-0 nav-y-sm font-size-1 pl-3 flex-column">
<li class="nav-item"><a class="nav-link" href="#">Sub-item 1</a></li>
<li class="nav-item"><a class="nav-link active" href="#">Sub-item 2</a></li>
<li class="nav-item"><a class="nav-link" href="#">Sub-item 3</a></li>
</ul>
</li>
<li class="nav-item"><a class="nav-link" href="#">Sub-item 3</a></li>
</ul>
</li>
<li class="nav-item"><a class="nav-link" href="#">Item 3</a></li>
</ul>