fix(docs-infra): make top-nav consistent across all marketing pages (#40881)

Previously, some of the marketing pages had different styles for the
top-nav than others (even if they had the same layout and
blue-background header). More specifically, the top-nav had a box-shadow
and it was absolutely positioned on some marketing pages, while it had
no box-shadow and was statically positioned on others.

This commit makes the appearance of marketing pages wrt the top-nav
consistent across all marketing pages by changing the styles for the
remaining pages:
- Contributors (`/about`)
- Contribute (`/contribute`)
- Press kit (`/presskit`)

Before: ![contribute topnav shadow before][1]
After: ![contribute topnav shadow after][2]

[1]: https://user-images.githubusercontent.com/8604205/107984898-a6aaee00-6fd1-11eb-8bf3-79393c8983ff.png
[2]: https://user-images.githubusercontent.com/8604205/107984900-a7438480-6fd1-11eb-8d9b-a643d69ab692.png

PR Close #40881

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

@@ -14,7 +14,7 @@ mat-toolbar.app-toolbar {
   box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.3);
 
   // HOME PAGE OVERRIDE: TOPNAV TOOLBAR
-  aio-shell.page-home & {
+  .page-home & {
     background-color: $blue;
 
     @media (min-width: 481px) {
@@ -26,10 +26,13 @@ mat-toolbar.app-toolbar {
   }
 
   // MARKETING PAGES OVERRIDE: TOPNAV TOOLBAR AND HAMBURGER
-  aio-shell.page-home &,
-  aio-shell.page-features &,
-  aio-shell.page-events &,
-  aio-shell.page-resources & {
+  .page-about &,
+  .page-contribute &,
+  .page-events &,
+  .page-features &,
+  .page-home &,
+  .page-presskit &,
+  .page-resources & {
     box-shadow: none;
 
     // FIXED TOPNAV TOOLBAR FOR SMALL MOBILE