Adds `Not found (404)` to `indexableStatuses`

[francofantini]

This PR adds Not Found (404) to the list of indexableStatuses. This change ensures that pages returning a 404 status are correctly recognized and processed within the script's indexing logic, enhancing the accuracy of indexing operations for sites with varying page statuses.

Please let me know if you have feedback.

[goenning]

I'm not sure if this is a good move, requesting indexing of a page that doesn't exist will probably just result in spam