layouts: move `.Site` (deprecated) to `hugo`

Co-Authored-by: iGor milhit <igor@milhit.ch>

layouts/partials/head.html

@@ -3,7 +3,7 @@
   <meta name="viewport" content="width=device-width">
   <title>{{ .Title  }} - {{ .Site.Title }}</title>
   <link rel="canonical" href="{{ .Permalink  }}">
-  {{- if .Site.IsServer -}}
+  {{- if hugo.IsServer -}}
   {{- $options := (dict "targetPath" "styles.css" "enableSourceMap" true) -}}
   {{- $styles := resources.Get "scss/main.scss" | toCSS $options }}
   <link rel="stylesheet" href="{{ $styles.Permalink }}" media="screen">