Update Doxygen Documentation

 * Update README with notification about dropping support
   for Ubuntu 16.04
 * Update cross-compile guide

Signed-off-by: Jan Eilers <jan.eilers@arm.com>
Change-Id: I10b89ba09bc5737012557eacdeaacc620ccdb8a9
diff --git a/21.02/_dot_serializer_8cpp_source.xhtml b/21.02/_dot_serializer_8cpp_source.xhtml
index 61cdd85..46db21a 100644
--- a/21.02/_dot_serializer_8cpp_source.xhtml
+++ b/21.02/_dot_serializer_8cpp_source.xhtml
@@ -131,7 +131,7 @@
 <div id="nav-path" class="navpath"><!-- id is needed for treeview function! -->
   <ul>
     <li class="navelem"><a class="el" href="dir_68267d1309a1af8e8297ef4c3efbcdba.xhtml">src</a></li><li class="navelem"><a class="el" href="dir_5d9c764a006d252ff232a5efbbd99a46.xhtml">armnnUtils</a></li><li class="navelem"><a class="el" href="_dot_serializer_8cpp.xhtml">DotSerializer.cpp</a></li>
-    <li class="footer">Generated on Thu Feb 25 2021 17:27:50 for ArmNN by
+    <li class="footer">Generated on Fri Mar 19 2021 15:26:00 for ArmNN by
     <a href="http://www.doxygen.org/index.html">
     <img class="footer" src="doxygen.png" alt="doxygen"/></a> 1.8.13 </li>
   </ul>