Update notifications reusable from bell to inbox (#28802)

Co-authored-by: Siara <[email protected]>

Author: gonmmarques

assets/images/help/notifications/notifications-general-existence-indicator.png

@@ -1,3 +1,3 @@
-1. In the upper-right corner of any page, click {% octicon "bell" aria-label="You have (no) unread notifications" %}.
+1. In the upper-right corner of any page, click {% octicon "inbox" aria-label="You have (no) unread notifications" %}.
 
-   ![Screenshot of the right corner of the header of GitHub. A bell icon with a blue dot indicating unread notifications is outlined in dark orange.](/assets/images/help/notifications/notifications-general-existence-indicator.png)
+   ![Screenshot of the right corner of the header of GitHub. An inbox icon has a blue dot, indicating that there are unread notifications.](/assets/images/help/notifications/notifications-general-existence-indicator.png)