How to create a rule to send an email when someone creates content of selected type(s)?
I'm trying to create a rule (using the Rules module 8.x-3.x-dev) to send an email when someone creates content that is of the type Announcement or Article

To my knowledge, the current answer to this question is a no go: You can't do that yet with the most recent Drupal 8 version of the Rules module.
Refer to #d8rules status update November 2017 to get a better understanding of the current status of Rules for D8. Here are some quotes from it:


... Fago can’t dedicate time required to develop the module further. What Rules is really missing at the moment is development capacity to help finish the last milestone.

... Even if we got more funding today, fago the current principal maintainer of the Rules module wouldn’t have enough capacity to do the work himself.



One never realizes what has been done, only what remains to be done ... So start here if you wonder how to contribute.
Plan B
While waiting for Rules to become more mature, you can use the Admin Content Notification module as a temporary alternative (which does have an official D8 release already). Some details about it (from its project page):

... can be used for receive email notification when a new content of any content type will be created on your site. Although one can use rules for the same functionality but it's not good to use rules only for notification as it is a huge module, so we can use this module for small websites.

The &quot;...Although one can use rules for the same functionality ...&quot; part of this clearly does not apply to the D8 version of Rules (yet).