refactor(docs-infra): scope footer styles to the `<aio-footer>` element (#40704)

This commit ensures that all styles for the page footer (which are
defined in `_footer.scss`) only apply to elements inside an
`<aio-footer>` element. This will prevent the styles accidentally taking
effect on a different part of the app.

PR Close #40704

aio/src/styles/1-layouts/_footer.scss

@@ -15,7 +15,6 @@ footer {
       color: $white;
       max-width: 50em;
     }
-  }
 
     .footer-block {
       margin: 0 24px;
@@ -92,6 +91,7 @@ footer {
         }
       }
     }
+  }
 
   &::after {
     content: "";