Issue #3515924: Add "check your email" page.

Closes #3515924

src/Controller/ThankYou.php

+<?php
+
+namespace Drupal\verify_email\Controller;
+
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\verify_email\Entity\VerifyEmail;
+
+/**
+ * Controller for handling the thank you page.
+ */
+class ThankYou extends ControllerBase {
+
+  /**
+   * Renders the thank you page.
+   *
+   * @param \Drupal\verify_email\Entity\VerifyEmail|null $entity
+   *   The verify email entity passed from the route.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function thankYouPage(VerifyEmail|null $entity = NULL) {
+    if ($entity) {
+      // Display the custom message.
+      $message = t('Please check your email and click on the included link to verify your email address.');
+
+      return [
+        '#markup' => $message,
+        '#title' => t('Thank you'),
+      ];
+    }
+
+    // Fallback message in case entity doesn't exist.
+    return [
+      '#markup' => t('Something has gone wrong. Please try submitting your email address again.'),
+      '#title' => t('Sorry'),
+    ];
+  }
+
+}