[docs] Round the devicePixelRatio

Google Analytics doesn't provide any tool to group close values.
We have to group the value ourselves to reduce the cardinality of the possible values this
dimension can take.
After 24 hours, we had 185 different values #21209 (comment).
After 4 days, we had 357 different values.

My hope is that we will have most of the values (> 1%) on less than 40 possible values).
This will make studying the result easier.

docs/pages/_app.js

@@ -160,7 +160,7 @@ function Analytics() {
      * Adjusted to track 3 or more different ratios
      */
     function trackDevicePixelRation() {
-      window.ga('set', 'dimension3', window.devicePixelRatio);
+      window.ga('set', 'dimension3', Math.round(window.devicePixelRatio * 10) / 10);
 
       matchMedia = window.matchMedia(`(resolution: ${window.devicePixelRatio}dppx)`);
       // Need to setup again.