Build the page scaffolding with CSS grid

Grid columns are symmetrical. From the outside in, they are:

- outer margin
- inner margin
- wide gutter
- inner gutter
- main content

assets/css/099_content.css

@@ -21,6 +21,11 @@
 .content-title {
   margin-block: var(--space-xl) var(--space-l);
 
+  /* margin-block-start is handled by the grid */
+  &:first-child, &:first-child .title {
+    margin-block-start: 0;
+  }
+
   .title:has(+ .subtitle) {
     margin-block-end: var(--space-xs);
   }