Fix <hr> styling (#2050)

* Add simple <hr> styling

* Run prettier

Author: reakaleek

src/Elastic.Documentation.Site/Assets/markdown/hr.css

@@ -0,0 +1,7 @@
+@layer components {
+	.markdown-content {
+		hr {
+			@apply bg-grey-20 mt-9 mb-8 h-[1px] border-0;
+		}
+	}
+}

src/Elastic.Documentation.Site/Assets/styles.css

@@ -18,6 +18,7 @@
 @import './markdown/definition-list.css';
 @import './markdown/images.css';
 @import './markdown/image-carousel.css';
+@import './markdown/hr.css';
 @import './modal.css';
 @import './archive.css';
 @import './markdown/stepper.css';