The default Evergreen MediaWiki skin (Chameleon) is a bootstrap-based skin.
Understanding the Bootstrap 3.0 classes available for grid-based page layouts could help you better understand how the pages are structured.
Create a Two Column Layout Help Wiki default article
Create a main content + sidebar layout
<div class="container-fluid"> <div class="row"> <div class="col-md-7 "> <div class="lead">Lead text goes here </div> </div><!-- end col-md-7 --> <div class="col-md-1"></div> <div class="col-md-3 sidebar"> *[[sidebar links can go here]] </div> <!-- end col-md-3--> </div> <!-- end row --> </div> <!-- end container-->