docs(aio): use h1 rather than divs and hide heading anchors

In the marketing pages we do not want to show heading anchors on hover.
Previously, this was achieved by using div rather than heading elements.
Now we can use semantically accurate headings while hiding the anchor.

Closes #17244
Closes #17264

aio/content/marketing/contribute.html

@@ -1,14 +1,14 @@
 <header class="marketing-banner">
-  <div class="banner-headline no-toc">Contribute to Angular</div>
+  <h1 class="banner-headline no-toc no-anchor">Contribute to Angular</h1>
 </header>
 
 <article class="contribute-container">
-  <h2>Angular Projects</h2>
+  <h2 class="no-anchor">Angular Projects</h2>
 
   <p>We'd love for you to contribute to our source code and to make Angular projects even better.</p>
 
   <div class="l-sub-section">
-    <h3>Angular</h3>
+    <h3 class="no-anchor">Angular</h3>
 
     Angular is a next generation mobile and desktop application development platform.
 
@@ -16,7 +16,7 @@
   </div>
 
   <div class="l-sub-section">
-    <h3>Angular Material</h3>
+    <h3 class="no-anchor">Angular Material</h3>
 
     Our goal is to deliver a lean, lightweight set of Angular-based UI elements that implement the material design specification for use in Angular single-page applications (SPAs).
 
@@ -24,7 +24,7 @@
   </div>
 
   <div class="l-sub-section">
-    <h3>AngularFire</h3>
+    <h3 class="no-anchor">AngularFire</h3>
 
     AngularFire is the officially supported Angular binding for Firebase. Firebase is a full backend so you don't need servers to build your Angular app.
 

aio/content/marketing/events.html

@@ -1,5 +1,5 @@
 <header class="marketing-banner">
-  <div class="banner-headline no-toc">Events</div>
+  <h1 class="banner-headline no-toc no-anchor">Events</h1>
 </header>
 
 <article class="l-content ">

aio/content/marketing/features.html

@@ -1,5 +1,5 @@
 <header class="marketing-banner">
-  <div class="banner-headline no-toc">Features</div>
+  <h1 class="banner-headline no-toc no-anchor">Features</h1>
 </header>
 
 <article class="l-content ">

aio/content/marketing/presskit.html

@@ -1,5 +1,5 @@
 <header class="marketing-banner">
-  <div class="banner-headline no-toc">Press kit</div>
+  <h1 class="banner-headline no-toc no-anchor">Press kit</h1>
 </header>
 
 <article class="presskit-container">

aio/content/marketing/resources.html

@@ -1,5 +1,5 @@
 <header class="marketing-banner">
-  <div class="banner-headline no-toc">Explore Angular Resources</div>
+  <h1 class="banner-headline no-toc no-anchor">Explore Angular Resources</h1>
 </header>
 
 <article>