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_site_header.css

@@ -44,11 +44,3 @@
     }
   }
 }
-
-.site-header__container,
-.site-footer__container {
-  /* Center the content and ensure a minimum margin. The calculation in the
-   * second argument does what `auto` would do if it could be used in max(). */
-  margin-inline: max(var(--space-m), 50% - var(--content-width) / 2);
-  max-width: var(--content-width);
-}