Fix off-by-one rendering of all highlighted lines in the docs! (#4221)

Author: nathan-contino

layouts/_default/_markup/render-codeblock.html

@@ -3,7 +3,14 @@
 {{- $attributes = merge $attributes (dict "class" (delimit $classes " ")) }}
 <pre
   {{- range $k, $v := $attributes }}
-    {{- printf " %s=%q" $k $v | safeHTMLAttr }}
+    {{- if eq $k "data-line-offset" -}}
+      data-line-offset="{{ int $v | add -1 }}"
+    {{- else -}}
+      {{- printf " %s=%q" $k $v | safeHTMLAttr -}}
+    {{- end -}}
+  {{- end -}}
+  {{- if not (isset $attributes "data-line-offset") -}}
+    data-line-offset="0"
   {{- end -}}
 >
 <code>