Add "default section content" example to docs

[reinink]

Currently the docs don't explicitly show how to setup default section content. Add the following example to the sections and/or inheritance page.

<div id="sidebar">
    <?php if ($this->section('sidebar')): ?>
        <?=$this->section('sidebar')?>
    <?php else ?>
        <?=$this->fetch('default-sidebar')?>
    <?php endif ?>
</div>

[reinink]

Also make note of the new default section value, which can be helpful is certain situations:

<title><?=$this->section('pagetitle', 'Site name')?></title>