Improve accessibility and polish page styling

app/not-found.tsx

@@ -3,7 +3,7 @@ import Link from "next/link";
 export default async function NotFound() {
   const { t } = await getI18n();
   return (
-    <main className="section page-content">
+    <main className="section page-content" id="main-content" tabIndex={-1}>
       <span className="eyebrow">{t("UNKNOWN COORDINATES")}</span>
       <h1>{t("Uncharted territory.")}</h1>
       <p>{t("This map or page isn’t available.")}</p>