Sticky breadcrumb and sidebar navs

Both the breadcrumb (parent) and sidebar (sibling) navs provide key context while reading the page. Making them sticky (and optionally giving the sidebar nav its own scroll context) makes their context available to the whole page, not just the start.

styles/theme-base.css

@@ -532,6 +532,10 @@ pre.info {
     padding:.75rem 1.5rem 1.5rem;
     -moz-box-sizing:border-box;
     box-sizing:border-box;
+    position: sticky;
+    top: 3rem;
+    max-height: calc(100vh - 3rem);
+    overflow: auto;
 }
 #layout-content {
   padding:1.5rem;
@@ -1352,6 +1356,10 @@ div.soft-deprecation-notice blockquote.sidebar {
 @media (min-width: 768px) {
   #breadcrumbs {
     display:block;
+    position: sticky;
+    top: 0px;
+    background: var(--dark-grey-color);
+    z-index: 1;
   }
   #intro .background,
   aside.tips,
@@ -1363,6 +1371,7 @@ div.soft-deprecation-notice blockquote.sidebar {
   #layout-content {
     float:left;
     width:75%;
+    border-top-left-radius: 1rem;
   }
 }