feat(a11y): add more detailed desc to alt text of the image

Author: robinboehm

src/app/book/book-detail/book-detail.component.html

@@ -8,7 +8,8 @@
     mat-card-image
     src="/assets/covers/{{ book.isbn }}.jpg"
     class="book-cover"
-    alt="book cover"
+    alt="book cover for the book {{book.title}} by {{book.author}}"
+    tabindex="0"
   />
   <mat-card-content>
     <p>