homepage: use srcset.

This should reduce the amount of bytes for non-2x displays.

site/index.html

@@ -6,7 +6,9 @@ layout: home
   <div class="container">
     <div class="row align-items-center">
       <div class="col-6 mx-auto col-md-6 order-md-2">
-        <img class="img-fluid mb-3 mb-md-0" src="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack.png" alt="" width="1024" height="860">
+        <img class="img-fluid mb-3 mb-md-0" srcset="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack.png,
+                                                    {{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack@2x.png 2x"
+                                            src="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-stack.png" alt="" width="512" height="430">
       </div>
       <div class="col-md-6 order-md-1 text-center text-md-left pr-md-5">
         <h1 class="mb-3 bd-text-purple-bright">Bootstrap</h1>
@@ -80,7 +82,9 @@ gem install bootstrap -v {{ site.current_ruby_version }}
     <p>
       Take Bootstrap 4 to the next level with official premium themes—toolkits built on Bootstrap with new components and plugins, docs, and build tools.
     </p>
-    <img class="img-fluid mt-3 mx-auto" src="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-themes.png" alt="Bootstrap Themes" width="1024" height="388">
+    <img class="img-fluid mt-3 mx-auto" srcset="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-themes.png,
+                                                {{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-themes@2x.png 2x"
+                                        src="{{ site.baseurl }}/docs/{{ site.docs_version }}/assets/img/bootstrap-themes.png" alt="Bootstrap Themes" width="500" height="200">
     <hr class="half-rule">
     <a href="{{ site.themes }}/" class="btn btn-outline-primary">Browse themes</a>
   </div>