fix(docs-infra): match card grid row gap to column gap

The `:not(.docs-card-container) .docs-card` selector was meant to give
standalone cards a vertical margin, but no element has the
docs-card-container class, so it matched every card. Cards in a grid
without a header picked up the 1rem margin on top of the 1.25rem grid
gap, leaving rows 52px apart while columns are 20px apart (for example on
/reference/migrations, /guide/i18n and /roadmap).

Apply the margin only to cards that are not direct children of a card
grid.

adev/shared-docs/styles/docs/_card.scss

@@ -122,7 +122,7 @@
     }
   }
 
-  :not(.docs-card-container) .docs-card {
+  :not(.docs-card-grid) > .docs-card {
     margin: 1rem 0;
   }