Put less emphasis on "ES5"/"ES6" in the README and other documentation

- Don't explicitly mention "ES5" in https://mozilla.github.io/pdf.js/getting_started/#download, since that's no longer accurate. As-is, this will more than likely give users the wrong impression regarding our *actual* browser support.

 - Reword the "Getting the Code"-section of the README, since an ES6-compatible browser is no longer sufficient for development purposes given our usage of modern ECMAScript features.

 - Update the "Building PDF.js"-section of the README, to explicitly mention both the `gulp generic` and `gulp generic-es5` commands.

Hopefully this will help reduce *some* user confusion, but we should perhaps also consider changing the "-es5"-suffix in our build-scripts and build-folders. (That would however require coming up with a "better" suffix, which might not be trivial.)

docs/contents/getting_started/index.md

@@ -49,7 +49,7 @@ Before downloading PDF.js please take a moment to understand the different layer
     </span>
   </div>
   <div class="col-md-4">
-    <h3>Prebuilt (ES5-compatible)</h3>
+    <h3>Prebuilt (for older browsers)</h3>
     <p>
       Includes the generic build of PDF.js and the viewer.
     </p>