Add CRUD about books

Following step https://api-platform.com/docs/distribution/#bringing-your-own-model

Specs from api/src/Entity/Greeting.php had to be removed too.

api/src/Entity/Book.php

@@ -0,0 +1,26 @@
+<?php
+// api/src/Entity/Book.php
+namespace App\Entity;
+use ApiPlatform\Metadata\ApiResource;
+use Doctrine\Common\Collections\ArrayCollection;
+/** A book. */
+#[ApiResource]
+class Book
+{
+    /** The ID of this book. */
+    private ?int $id = null;
+    /** The ISBN of this book (or null if doesn't have one). */
+    public ?string $isbn = null;
+    /** The title of this book. */
+    public string $title = '';
+    /** The description of this book. */
+    public string $description = '';
+    /** The author of this book. */
+    public string $author = '';
+    /** The publication date of this book. */
+    public ?\DateTimeImmutable $publicationDate = null;
+    public function getId(): ?int
+    {
+        return $this->id;
+    }
+}