Extend grid view to the right to better fill the page

- Use a negative right margin to pull `.grid__wrapper` into the dead space left for a sidebar.
- Add sample documents to "portfolio" collection to test grid view
- Update CHANGELOG and history

_sass/minimal-mistakes/_archive.scss

@@ -96,6 +96,21 @@
    Grid view
    ========================================================================== */
 
+.archive {
+
+  .grid__wrapper {
+    /* extend grid elements to the right */
+
+    @include breakpoint($large) {
+      margin-right: -1 * $right-sidebar-width-narrow;
+    }
+
+    @include breakpoint($x-large) {
+      margin-right: -1 * $right-sidebar-width;
+    }
+  }
+}
+
 .grid__item {
   margin-bottom: 2em;