Improved positioning of back-to-top button

Signed-off-by: Jim Martens <github@2martens.de>

_assets/css/critical.scss

@@ -9,14 +9,40 @@
   border-bottom: 2px solid $light;
 }
 
+// back-to-top button
 #back-to-top {
   cursor: pointer;
-  position: fixed;
-  bottom: 20px;
-  right: 20px;
+  position: -webkit-sticky;
+  position: sticky;
+  bottom: 40px;
   display:none;
 }
 
+@include media-breakpoint-down(sm) {
+  #back-to-top {
+    position: fixed;
+    right: 20px;
+  }
+}
+
+@include media-breakpoint-only(md) {
+  #back-to-top {
+    margin-left: 95%;
+  }
+}
+
+@include media-breakpoint-only(lg) {
+  #back-to-top {
+    margin-left: 65%;
+  }
+}
+
+@include media-breakpoint-only(xl) {
+  #back-to-top {
+    margin-left: 60%;
+  }
+}
+
 $navbarHeight: 58px;
 
 .large {