My recommendation would be to create a category for this (“Announcements”) and make it read-only or read-and-reply-only for normal users.
Then, make new users watch that category by default (via site settings), and apply this for existing users. If you choose to allow replies, choose the Watching First Post notification level.
This way, whenever you make a new topic in that category, every user will get a notification by default. Users can opt out by changing the notification level for the category.
(Note that this will reduce the amount of digests your site sends out, because any mail Discourse sends to a user resets the digest period – so if a user has set digests to weekly, they will only get a digest if no mail has been sent to them for a week, including these announcements.)