documentation: Add contributing guide

doc/ns3_html_theme/ns3_doxy_header.html

@@ -68,15 +68,18 @@ $extrastylesheet
 		      <div id="mDocs"
 		        onmouseover="mcancelclosetime()" 
 		        onmouseout="mclosetime()">
-			<a id="ns3_wiki"
-			  href="http://www.nsnam.org/wiki"
-			  >Wiki</a><br/>
 			<a id="ns3_man"
 			    href="/docs/manual/html/index.html"
 			    >Manual</a><br/>
 			<a id="ns3_mod"
 			   href="/docs/models/html/index.html"
 			   >Models</a><br/>
+			<a id="ns3_con"
+			   href="/docs/contributing/html/index.html"
+			   >Contributing</a><br/>
+			<a id="ns3_wiki"
+			  href="http://www.nsnam.org/wiki"
+			  >Wiki</a><br/>
 		    </li>
 		    <li><span
 			onmouseover="mopen('mDev')" 
@@ -89,7 +92,9 @@ $extrastylesheet
 			   href="/docs/doxygen/index.html"
 			   >API</a><br/>
 			<a id="ns3_bugs"
-			   href="http://www.nsnam.org/bugzilla/">Bugs</a>
+			   href="https://gitlab.com/nsnam/ns-3-dev/-/issues">Bugs</a><br/>
+			<a id="ns3_merge"
+			   href="https://gitlab.com/nsnam/ns-3-dev/-/merge_requests">Merge Requests</a>
 		    </li>
 		  </ul>
 		</div>