Use flexbox instead of CSS grid

This makes it easier to customize card components for future work. The
side effect is that the last row is centered, which was suggested
elsewhere¹.

¹ #878 (comment)

static-site/src/components/Showcase/index.tsx

@@ -111,9 +111,9 @@ const PreviewOverlay = styled.div`
 
 const CardsContainer = styled.div`
   /* background-color: #ffeab0; */
-  display: grid;
-  grid-template-columns: repeat(auto-fit, minmax(${cardWidthHeight}px, max-content));
-  grid-gap: 1%;
+  display: inline-flex;
+  flex-wrap: wrap;
+  gap: 10px;
   overflow: hidden;
   justify-content: center;
 `;