Add POD for Unread Messages feature

Present the Unread Messages Notification and describe how to enable it
at user and admin levels.

Also describe how to use the Unread Messages column in the ticket
search results.

Finally describe how to search tickets that have unread messages.

docs/unread_messages.pod

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+=head1 Unread Messages
+
+RT can show a notification to users when a ticket has unread messages and
+helps them keep track of their unread messages.
+
+It also provides ways to search for tickets with or without unread
+messages per user, facilitating efficient ticket management.
+
+=head1 Enabling Unread Messages Notifications
+
+RT users can enable or disable the Unread Messages Notification on their
+preferences page:
+
+    Logged in as ... > Settings > Preferences > Notify me of unread messages
+
+After enabling this option, users will see a notification on the top of the
+page when they have unread messages in a ticket. The notification will
+disappear when the user marks the messages as seen.
+
+=for html <img alt="Unread Messages Notification" src="images/unread-notification.png">
+
+=for :text [Unread Messages Notification F<docs/images/unread-notification.png>]
+
+=for :man [Unread Messages Notification F<docs/images/unread-notification.png>]
+
+=head2 Enabling Unread Messages Notifications for All Users
+
+The Unread Messages Notification can be enabled for all users by default.
+To do so, set the L<RT_Config.pm/"$ShowUnreadMessageNotifications">
+configuration in the RT_SiteConfig.pm file:
+
+    Set($ShowUnreadMessageNotifications, 1);
+
+=head1 Adding the Unread Messages Column to Search Results
+
+In previous versions of RT, the 'UpdateStatus' column was used to display
+Yes/No for unread messages. In newer versions, the 'UnreadMessages' column
+is utilized instead, showing a count of unread messages by the current user.
+
+To add this column to Search Results or a Dashboard:
+
+    - Navigate to the desired search or dashboard.
+    - Click on 'Edit Search'.
+    - Add the 'UnreadMessages' column in the "Display Columns" section.
+
+=for html <img alt="Unread Messages Column" src="images/unread-messages-column.png">
+
+=for :text [Unread Messages Column F<docs/images/unread-messages-column.png>]
+
+=for :man [Unread Messages Column F<docs/images/unread-messages-column.png>]
+
+=head1 Searching for Tickets with and without Unread Messages
+
+Request Tracker introduces two new search terms for TicketSQL to facilitate
+searching based on unread messages:
+
+=over
+
+=item Search Term: HasUnreadMessages
+
+This search term is used to filter tickets that contain unread messages for
+indicated users. Example usage:
+
+    queue = 'General' AND Status = 'open' AND HasUnreadMessages = 'john'
+
+This query will retrieve tickets in the 'General' queue that are open and
+have unread messages for the user 'john'.
+
+You can also have an abstract search term that will look for unread messages
+of the current user:
+
+    queue = 'General' AND Status = 'open' AND HasUnreadMessages = '__CurrentUser__'
+
+=item Search Term: HasNoUnreadMessages
+
+This search term filters tickets that do not have any unread messages.
+Example usage:
+
+    queue = 'Support' AND HasNoUnreadMessages='__CurrentUser__'
+
+This query will fetch tickets in the 'Support' queue that do not have any
+unread messages for the current user.
+
+=back