fix(docs-infra): use a fixed top-menu on marketing pages as well (#41051)

Previously, in contrast to docs pages, marketing pages had a non-fixed
top-menu, which meant that the top-menu would scroll out of the viewport
along with the rest of the content. This had a couple of downsides:
- The UI was different between pages (i.e. different top-menu behavior
  on docs vs marketing pages).
- Since some of the marketing pages are long, it was not easy for people
  to navigate to a different page (i.e. they had to scroll all the way
  back up).

This commit improves the UX by using the same, fixed top-menu on all
pages, which restores consistency and allows the user to navigate around
more easily.

NOTE:
The old behavior (non-fixed top-menu) is kept on the homepage, since its
top-menu design in a little different than other pages (e.g. it uses a
transparent top-menu) and would not play well with a fixed top-menu.

PR Close #41051

aio/src/styles/1-layouts/_top-menu.scss

@@ -20,21 +20,13 @@ mat-toolbar.app-toolbar {
     @media (min-width: 481px) {
       &:not(.transitioning) {
         background-color: transparent;
+        box-shadow: none;
+        position: absolute;
         transition: background-color 0.2s linear;
       }
     }
   }
 
-  // MARKETING PAGES OVERRIDE: TOPNAV TOOLBAR AND HAMBURGER
-  @include marketing-pages($nestParentSelector: true) {
-    box-shadow: none;
-
-    // FIXED TOPNAV TOOLBAR FOR SMALL MOBILE
-    @media (min-width: 481px) {
-      position: absolute;
-    }
-  }
-
   // DOCS PAGES OVERRIDE: HAMBURGER
   @include docs-pages($nestParentSelector: true) {
     @media (min-width: $showTopMenuWidth) {