Add `Module::getNotifications()` to "Development" -> "Notifications" page

[marc-farre]

I suggest adding a mention on https://docs.humhub.org/docs/develop/notifications about the Module::getNotifications() method.
I can send a PR for it.

@luke- However, I don't understand why some module use it (e.g., the Messenger module here), whereas others don't (e.g., the Calendar module).

[luke-]

@marc-farre Would be great!

@yurabakhtin Can you help?