Fix z-indexes of code example feature and cleanup its CSS #103722
                
     Merged
            
            
          
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When reviewing #103650, I realized that the
z-indexes of this feature were completely broken:This PR fixes it by reducing the value of value under the one used for
.popover(it could be completely removed but then it wouldn't be displayed as nicely).There was also a lot of duplicated CSS so I merged the rules.
r? @notriddle