Make sidebar sticky

Do this by making the contents area max out to 100vh so that it scrolls
independently of the sidebar.

src/assets/styles/base/_layout.scss

@@ -120,6 +120,7 @@ body {
   .technote-container {
     grid-template-columns: var(--tn-sidebar-width) 1fr;
     grid-template-rows: auto 1fr;
+    height: 100vh; /* Full height to allow for sticky sidebar */
   }
 
   .technote-logo-container {
@@ -139,6 +140,9 @@ body {
     grid-column: 2 / 3;
     grid-row: 1 / 3;
 
+    /* Allow overflow because .technote-container is full height */
+    overflow-y: auto;
+
     /* Create a grid within article to constrain content but allow
        full-bleed blocks.
      */