Revise page header styles

- more compact headers when announcements are showing
- when no announcement showing, top level docs sections
  (eg Concepts, Tasks, Reference) have a heading in the top nav
- when announcement is showing, rely on the breadcrumb trail plus
  page introduction text for each top level section
- switch from plain black docs header to match other sections

layouts/docs/docsportal_home.html

@@ -1,7 +1,4 @@
 {{ define "main" }}
-    {{ if not .Params.notitle }}
-	 <h1>{{ .Title }}</h1>
-    {{ end }}
     {{ template "docs-portal-content" . }}
 {{ end }}
 
@@ -19,4 +16,11 @@
     </div>
   </div>
 </section>
-{{ end }}
+{{ end }}
+{{ define "hero" }}
+    <section class="header-hero text-white pb-4 light-text ">
+    {{ if not .Params.notitle }}
+    <h1>{{ .Title }}</h1>
+    {{ end }}
+    </section>
+{{ end }}