docs: a few minor docs improvements (#26404)

html tags like b and i are update and add the alt attribute in the img tag

(cherry picked from commit 40df237)

src/components-examples/cdk/dialog/cdk-dialog-overview/cdk-dialog-overview-example.html

@@ -7,6 +7,6 @@
     <button (click)="openDialog()">Pick one</button>
   </li>
   <li *ngIf="animal">
-    You chose: <i>{{animal}}</i>
+    You chose: <em>{{animal}}</em>
   </li>
 </ol>

src/components-examples/cdk/listbox/cdk-listbox-compare-with/cdk-listbox-compare-with-example.html

@@ -18,5 +18,5 @@
   <!-- #enddocregion listbox -->
 </div>
 <p *ngIf="appointment[0]">
-  Your appointment  is scheduled for <b>{{formatAppointment() | json}}</b>&nbsp;
+  Your appointment  is scheduled for <strong>{{formatAppointment() | json}}</strong>&nbsp;
 </p>

src/components-examples/material/card/card-media-size/card-media-size-example.html

@@ -17,7 +17,7 @@
     <mat-card-title-group>
       <mat-card-title>Shiba Inu</mat-card-title>
       <mat-card-subtitle>Medium</mat-card-subtitle>
-      <img mat-card-md-image src="https://material.angular.io/assets/img/examples/shiba2.jpg" >
+      <img mat-card-md-image src="https://material.angular.io/assets/img/examples/shiba2.jpg"  alt="Image of a Shiba Inu">
     </mat-card-title-group>
   </mat-card-header>
   <mat-card-content>