added css class boost-logo

[SVN r22351]

rst.css

@@ -284,3 +284,8 @@ h1 tt, h2 tt, h3 tt, h4 tt, h5 tt, h6 tt {
 
 ul.auto-toc {
   list-style-type: none }
+
+img.boost-logo {
+  border: none;
+  vertical-align: middle
+}