fix: use more semantic element for page header description

Use a paragraph element instead of a div, which is a better semantic
choice for a subtitle. This is a small improvement to the accessibility
tree for important text elements at the top of the page.

Author: jawinn

blocks/page-header/page-header.css

@@ -53,7 +53,7 @@
 
   .page-header__description,
   .page-header__byline {
-    margin-block-end: 1.25rem;
+    margin-block: 0 1.25rem;
 
     @media (min-width: 48rem) {
       margin-block-end: 1.875rem;

blocks/page-header/page-header.js

@@ -26,7 +26,7 @@ export default async function decorate(block) {
 
   // if there is a description, add it as an h2
   if (pageHeaderData.description) {
-    const pageDescription = document.createElement("div");
+    const pageDescription = document.createElement("p");
     pageDescription.classList.add("page-header__description");
 
     // if there is a byline, the description uses