doc: put drop down menus always on top

doc/ns3_html_theme/layout.html

@@ -96,9 +96,11 @@
 			 href="/docs/doxygen/html/index.html"
 			 >API</a><br/>
 		      <a id="ns3_bugs"
-		       href="https://gitlab.com/nsnam/ns-3-dev/-/issues">Bugs</a>
+		       href="https://gitlab.com/nsnam/ns-3-dev/-/issues"
+			 >Issue Tracker</a><br/>
 		      <a id="ns3_merge"
-		       href="https://gitlab.com/nsnam/ns-3-dev/-/merge_requests">Merge Requests</a>
+		       href="https://gitlab.com/nsnam/ns-3-dev/-/merge_requests"
+			 >Merge Requests</a><br/>
 	        </li>
 	      </ul>
 	    </div>

doc/ns3_html_theme/ns3_doxy_header.html

@@ -92,9 +92,11 @@ $extrastylesheet
 			   href="/docs/doxygen/index.html"
 			   >API</a><br/>
 			<a id="ns3_bugs"
-			   href="https://gitlab.com/nsnam/ns-3-dev/-/issues">Bugs</a><br/>
+			   href="https://gitlab.com/nsnam/ns-3-dev/-/issues"
+			   >Issue Tracker</a><br/>
 			<a id="ns3_merge"
-			   href="https://gitlab.com/nsnam/ns-3-dev/-/merge_requests">Merge Requests</a>
+			   href="https://gitlab.com/nsnam/ns-3-dev/-/merge_requests"
+			   >Merge Requests</a><br/>
 		    </li>
 		  </ul>
 		</div>

doc/ns3_html_theme/static/ns3_stylesheet.css

@@ -85,6 +85,9 @@ div.sphinxsidebar a {
   background-repeat:repeat;
   border-bottom: 1px solid #5B5B5B;
   color: white;
+  position: relative;
+  /* Doxygen tab bar ("Main Page", "Related Pages"..) is at z: 9999 */
+  z-index:10000;
 }
 
 #projectlogo {