chore(layouts/index): use relURL for css background

because netlify optimization requires this, see

https://community.netlify.com/t/support-guide-does-netlify-optimize-every-single-asset-my-site-uses/152

layouts/index.html

@@ -1,11 +1,10 @@
 {{ define "main" }}
 {{ $headless := .Site.GetPage "/homepage" }}
 {{ $sections := $headless.Resources.ByType "page" }}
-<header id="site-head" {{ with .Params.header_image }}style="background-image: url({{ . | absURL }})"{{ end }}>
+<header id="site-head" {{ with .Params.header_image }}style="background-image: url({{ . | relURL }})"{{ end }}>
     <div class="vertical">
         <div id="site-head-content" class="inner">
 
-            
             {{ with .Params.header_headline }}<h1 class="blog-title">{{ . }}</h1>{{ end }}
             {{ with .Params.header_subheadline }}<h2 class="blog-description">{{ . }}</h2>{{ end }}