Doc: CSS: Update selectors for target highlighting

Certain targets on the generated HTML still use the obsolete
<a name=""> method of creating an anchor. Add a rule for a
replacement <span> element, using the id attribute.

This ensures that the highlighting animations continue to work
after the use of obsolete HTML is dropped from QDoc.

Change-Id: I2b70470b08d0e0f70702ad2820f4e065f2c08ff5
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

doc/global/template/style/online.css

@@ -1213,7 +1213,7 @@ pre:hover>.copy_text {
     width:initial
   }
 }
-[id]:target,[name]:target,[name]:target+* {
+[id]:target,[name]:target,[name]:target+*,span:target+* {
   -webkit-animation:highlighter 3s;
   animation:highlighter 3s
 }