Add back-to-top link in layout template (#470)

Added a back-to-top link for improved navigation.

Author: salomegarnier

sass_source/sass/_page-layout.scss

@@ -367,3 +367,10 @@ html body {
     text-decoration: none;
   }
 }
+
+.page__back-to-top-container {
+  width: 100%;
+  display: flex;
+  justify-content: flex-end;
+  margin-top: 1rem;
+}

source/_templates/layout.html

@@ -27,6 +27,12 @@
           <div class="clearer"></div>
         </section>
 
+        <br>
+
+        <div class="page__back-to-top-container">
+          <a href="#top" class="page__back-to-top">Back to top</a>
+        </div>
+
         {% if pagename not in ["index", "404", "search"] %}
           <section class="page__feedback">
             <h1>Was this article useful?</h1>