Returns a number, which represents the nesting depth of the page.
<select onchange="window.location.href = this.value">
<?php foreach($site->index() as $p): ?>
<option value="<?php echo $p->url() ?>">
<?php echo str_repeat('-', $p->depth()) ?>
<?php echo html($p->title()) ?>
</option>
<?php endforeach ?>
</select>