added installer download option

- new download card
- added buttons for each release file
- improved general responsiveness

_includes/contribute.html

@@ -6,7 +6,7 @@
                 {{ page.content | markdownify }}
                 <div class="flex-box-row">
                     {% for contribute in site.data.contribute %}
-                        <div class="col-md-4 flex-box-col card hover-card dark-text">
+                        <div class="col-md-4 col-sm-5 flex-box-col card hover-card dark-text">
                           <div class="flex-box-row">
                             <div class="col-md-3 flex-box-col">
                               <span class="contribute-icon fa fa-{{ contribute.icon }} flex00a" aria-hidden="true"></span>