Hi democloud,
Add another ContentPlaceHolder above the side bar menu for adding the contentpage menu with it.
Check this example. Now please take its reference and correct your code.
HTML
Master Page
<html>
<head runat="server">
<title></title>
<!-- CSS -->
<link href='https://fonts.googleapis.com/css?family=Roboto:400,100,100italic,300,300italic,400italic,500,500italic,700,700italic,900,900italic' rel='stylesheet' type='text/css' />
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css" />
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" />
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body class="archive">
<form id="form1" runat="server">
<!-- Header -->
<header class="hero overlay">
<div class="masthead single-masthead">
<div class="container">
<div class="row">
</div>
</div>
</div>
</header>
<div class="container">
<div class="row">
<div class="col-lg-8">
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</div>
<div class="col-lg-4">
<div class="sidebar">
<div class="pt-50">
<div class="widget fix widget_categories">
<asp:ContentPlaceHolder ID="ContentPlaceHolder2" runat="server">
</asp:ContentPlaceHolder>
<span class="icon icon-folder"></span>
<h4>Popular Knowledgebase Topics</h4>
<ul>
<li><a href="index.aspx">Modules </a></li>
<li><a href="Admin.aspx">Admin </a></li>
<li><a href="Staff_Mngmt.aspx">Staff Management </a></li>
<li><a href="Inventory.aspx">Inventory</a></li>
<li><a href="Quality.aspx">Quality </a></li>
</ul>
</div>
</div>
</div>
</div>
</div>
</div>
<footer>
<div class="container">
<div class="row">
<div class="col-md-6"></div>
<div class="col-lg-6 col-md-4 col-sm-12">
<div class="copyright">
</div>
</div>
</div>
</div>
</footer>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script type="text/javascript" src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/js/bootstrap.min.js"></script>
</form>
</body>
</html>
Content page
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
Content page content
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder2" runat="Server">
<h4>New Topics</h4>
<ul>
<li><a href="index.aspx">Test 1 </a></li>
</ul>
</asp:Content>
