Style: Make footer adaptive

- Set flex-flow of the list to "row wrap" to make the footer adaptive.
- Set display of the list to flex to make it clear we're using it.
  This is already set in Pico CSS so it isn't necessary, but I think
  it's nicer to have it set again explictly.
  See here for the relevant Pico CSS code:
  aaa1ffe05c/scss/components/_nav.scss (L20)

themes/ccchh/assets/sass/main.scss

@@ -30,6 +30,12 @@ header {
 body>footer {
   padding-top: var(--spacing);
   padding-bottom: var(--spacing);
+  nav {
+    ul {
+      display: flex;
+      flex-flow: row wrap;
+    }
+  }
 }
 
 body>main {