Add screenreader text to prev/next carousel buttons in examples to improve accessibility

Refs #13556.
Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]

docs/_includes/js/carousel.html

@@ -23,9 +23,11 @@
       </div>
       <a class="left carousel-control" href="#carousel-example-generic" role="button" data-slide="prev">
         <span class="glyphicon glyphicon-chevron-left"></span>
+        <span class="sr-only">Previous</span>
       </a>
       <a class="right carousel-control" href="#carousel-example-generic" role="button" data-slide="next">
         <span class="glyphicon glyphicon-chevron-right"></span>
+        <span class="sr-only">Next</span>
       </a>
     </div>
   </div><!-- /example -->
@@ -58,9 +60,11 @@
   <!-- Controls -->
   <a class="left carousel-control" href="#carousel-example-generic" role="button" data-slide="prev">
     <span class="glyphicon glyphicon-chevron-left"></span>
+    <span class="sr-only">Previous</span>
   </a>
   <a class="right carousel-control" href="#carousel-example-generic" role="button" data-slide="next">
     <span class="glyphicon glyphicon-chevron-right"></span>
+    <span class="sr-only">Next</span>
   </a>
 </div>
 {% endhighlight %}
@@ -104,9 +108,11 @@
       </div>
       <a class="left carousel-control" href="#carousel-example-captions" role="button" data-slide="prev">
         <span class="glyphicon glyphicon-chevron-left"></span>
+        <span class="sr-only">Previous</span>
       </a>
       <a class="right carousel-control" href="#carousel-example-captions" role="button" data-slide="next">
         <span class="glyphicon glyphicon-chevron-right"></span>
+        <span class="sr-only">Next</span>
       </a>
     </div>
   </div><!-- /example -->

docs/examples/carousel/index.html

@@ -113,8 +113,14 @@
           </div>
         </div>
       </div>
-      <a class="left carousel-control" href="#myCarousel" role="button" data-slide="prev"><span class="glyphicon glyphicon-chevron-left"></span></a>
-      <a class="right carousel-control" href="#myCarousel" role="button" data-slide="next"><span class="glyphicon glyphicon-chevron-right"></span></a>
+      <a class="left carousel-control" href="#myCarousel" role="button" data-slide="prev">
+        <span class="glyphicon glyphicon-chevron-left"></span>
+        <span class="sr-only">Previous</span>
+      </a>
+      <a class="right carousel-control" href="#myCarousel" role="button" data-slide="next">
+        <span class="glyphicon glyphicon-chevron-right"></span>
+        <span class="sr-only">Next</span>
+      </a>
     </div><!-- /.carousel -->
 
 

docs/examples/theme/index.html

@@ -633,9 +633,11 @@
         </div>
         <a class="left carousel-control" href="#carousel-example-generic" role="button" data-slide="prev">
           <span class="glyphicon glyphicon-chevron-left"></span>
+          <span class="sr-only">Previous</span>
         </a>
         <a class="right carousel-control" href="#carousel-example-generic" role="button" data-slide="next">
           <span class="glyphicon glyphicon-chevron-right"></span>
+          <span class="sr-only">Next</span>
         </a>
       </div>