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

assets/scss/_base.scss

@@ -810,11 +810,10 @@ section#cncf {
   }
 }
 
-.td-search {
-  header > .header-filler {
-    height: $hero-padding-top;
-    background-color: black;
-  }
+// Header filler size adjustment
+
+.header-hero.filler {
+  height: $hero-padding-top;
 }
 
 // Docs specific
@@ -859,17 +858,6 @@ section#cncf {
 
 /* DOCUMENTATION */
 
-body.td-documentation {
-    header > .header-filler {
-      height: $hero-padding-top;
-      background-color: black;
-    }
-    /* Special case for if an announcement is active */
-    header section#announcement ~ .header-filler {
-      display: none;
-    }
-}
-
 // nav-tabs and tab-content
 .nav-tabs {
   border-bottom: none !important;