Notifications: Started core user notification logic

Put together an initial notification.
Started logic to query and identify watchers.

app/Activity/Notifications/Messages/PageCreationNotification.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace BookStack\Activity\Notifications\Messages;
+
+use BookStack\Activity\Notifications\LinkedMailMessageLine;
+use BookStack\Entities\Models\Page;
+use Illuminate\Notifications\Messages\MailMessage;
+
+class PageCreationNotification extends BaseActivityNotification
+{
+    public function toMail(mixed $notifiable): MailMessage
+    {
+        /** @var Page $page */
+        $page = $this->detail;
+
+        return (new MailMessage())
+            ->subject("New Page: " . $page->getShortName())
+            ->line("A new page has been created in " . setting('app-name') . ':')
+            ->line("Page Name: " . $page->name)
+            ->line("Created By: " . $this->user->name)
+            ->action('View Page', $page->getUrl())
+            ->line(new LinkedMailMessageLine(
+                url('/preferences/notifications'),
+                'This notification was sent to you because :link cover this type of activity for this item.',
+                'your notification preferences',
+            ));
+    }
+}