Accessibility updates - main landmark roles (#17106)

* Removed multiple main tags from home page

Signed-off-by: chaitanyakaranam <chaitanya.karanam@yahoo.com>

* Removed multiple occurances of main in case-studies page

Signed-off-by: chaitanyakaranam <chaitanya.karanam@yahoo.com>

* Replaced css for main tag with .main-section class

Signed-off-by: ChaitanyaKaranam <karanam.krishna.25@gmail.com>

* Added main landmark role for all the pages

Signed-off-by: ChaitanyaKaranam <karanam.krishna.25@gmail.com>

* Force checks.

request-checks: true

assets/sass/_desktop.sass

@@ -95,7 +95,7 @@ $video-section-height: 550px
       right: -25px
 
   section, header, footer
-    main
+    .main-section
       max-width: $main-max-width
 
   header, #vendorStrip, #encyclopedia, #hero h1, #hero h5, #docs #hero h1, #docs #hero h5,
@@ -108,11 +108,11 @@ $video-section-height: 550px
 
   #home
     section, header, footer
-      main
+      .main-section
         max-width: 1000px
 
   #oceanNodes
-    main
+    .main-section
       position: relative
       max-width: 830px
 
@@ -172,7 +172,7 @@ $video-section-height: 550px
     background-image: url(../images/texture.png)
     background-color: $dark-grey
 
-    main
+    .main-section
       padding: 20px 0
 
     nav