Adjust sidebar nav sizing to account for rustdoc container padding.

This addresses some visible jank caused by the sidebar nav container
overflow changing the rendered size.

templates/style/rustdoc.scss

@@ -29,8 +29,9 @@ body.blur {
 
 // rustdoc overrides
 div.rustdoc {
+    $padding-x: 15px;
     font-family: $font-family-serif;
-    padding: 10px 15px 20px 15px;
+    padding: 10px $padding-x 20px;
     position: relative;
 
     @media (max-width: 700px) {
@@ -47,8 +48,13 @@ div.rustdoc {
         }
 
         @media (max-width: 700px) {
+            margin-left: -2 * $padding-x; // offset the additional padding added by the parent containers
+            width: calc(100% + #{4 * $padding-x});
+
             &.mobile {
                 top: $top-navbar-height;
+                margin-left: 0; // since the sidebar is now fixed position, remove the padding workaround
+                width: 100%;
 
                 .sidebar-elems.show-it {
                     top: 45px + $top-navbar-height;