Move background size to css (#2962)

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

_includes/banner.html

@@ -1,5 +1,5 @@
 <section class="os-banner">
-    <header class="banner-head" style="background-image: url('{{site.url}}{{site.baseurl}}/images/graphic-bg.png'); background-size: cover;">
+    <header class="banner-head" style="background-image: url('{{site.url}}{{site.baseurl}}/images/graphic-bg.png')">
         <div class="banner-text-div">
             <span class="banner-text-header">OpenSearch Documentation</span>
             <p class="banner-text-description">Learn to use OpenSearch, the highly scalable and extensible open-source software suite for search, analytics, observability, and other data-intensive applications.</p>

_sass/custom/custom.scss

@@ -419,6 +419,7 @@ p.label {
 
 .banner-head {
   @extend .panel;
+  background-size: cover;
 }
 
 .banner-text-div {