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

layouts/index.html

@@ -1,15 +1,15 @@
 {{ define "main" }}
 {{ .Content }}
 <section id="cncf">
-    <main>
+    <div class="main-section">
         <center>
         <p style="font-size: 20px">{{ T "main_cncf_project" | safeHTML }}
         </center>
-    </main>
+    </div>
 </section>
 
 <section id="kubeweekly">
-      <main>
+      <div class="main-section">
         <!-- Begin MailChimp Signup Form -->
         <link href="https://cdn-images.mailchimp.com/embedcode/horizontal-slim-10_7.css" rel="stylesheet" type="text/css">
         <style type="text/css">
@@ -35,7 +35,7 @@
 <br>
 <br>
         <!--End mc_embed_signup-->
-    </main>
+    </div>
 </section>
 {{ end }}