Add hgroup html tag (#1016)

The [<hgroup>](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/hgroup) HTML element represents a heading and related content. It groups a single <h1>–<h6> element with one or more <p>.

packages/html/src/elements.rs

@@ -359,6 +359,11 @@ builder_constructors! {
     /// element.
     header None {};
 
+    /// Build a
+    /// [`<hgroup>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/hgroup)
+    /// element.
+    hgroup None {};
+
     /// Build a
     /// [`<h1>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/h1)
     /// element.