[MINOR][DOC] Update header links and image files

Fix relative path when inside site

closes #1072

Author: j143

docs/_includes/header.html

@@ -35,10 +35,28 @@
         </div>
         <nav class="navbar-collapse collapse">
             <ul class="nav navbar-nav navbar-right">
-                <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/">Overview</a></li>
-                <li><a href="https://github.com/apache/systemds">GitHub</a></li>
                 <li class="dropdown">
-                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">API Docs<b class="caret"></b></a>
+                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Overview<b class="caret"></b></a>
+                    <ul class="dropdown-menu" role="menu">
+                        <li><b>Home:</b></li>
+                        <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/">Docs Home</a></li>
+                        <li class="divider"></li>
+                        <li><b>Running SystemDS:</b></li>
+                        <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/site/run">Standalone Guide</a></li>
+                        <li class="divider"></li>
+                        <li><b>Language Guides:</b></li>
+                        <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/site/dml-language-reference.html">DML Language Reference</a></li>
+                        <li class="divider"></li>
+                        <li><b>ML Algorithms:</b></li>
+                        <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/site/algorithms-reference.html">Algorithms Reference</a></li>
+                        <li class="divider"></li>
+                        <li><b>Other:</b></li>
+                        <li><a href="https://github.com/apache/systemds/blob/master/CONTRIBUTING.md">Contributing to SystemDS 🡕</a></li>
+                    </ul>
+                </li>
+                <li><a href="https://github.com/apache/systemds">GitHub 🡕</a></li>
+                <li class="dropdown">
+                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">API<b class="caret"></b></a>
                     <ul class="dropdown-menu" role="menu">
                         <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/api/java/">Java</a></li>
                         <li><a href=".{% if page.path contains 'site' %}/..{% endif %}/api/python/">Python</a></li>