Doc: Online style: CSS fixes for image alignment

- Remove duplicated rules
    - Add a rule for class centerAlign
    - Remove width restriction from paragraphs in table cells

These fixes enable proper horizontal center-align of images in
table cells.

Change-Id: I68a4863fe642a552260dfd6dbbb5d9675a8d2b4d
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

doc/global/template/style/online.css

@@ -1258,15 +1258,15 @@ li a.active {
   color:#404244;
   line-height:1.16em
 }
-.mainContent table p {
-  margin:0px;
-  padding:0px
-}
 .mainContent table p {
   margin:0px;
   padding:0px;
+  max-width:initial;
   min-height:2em
 }
+.mainContent table p.centerAlign {
+  text-align:center
+}
 .context h2 {
   font-size:2.1875em
 }