Trigger documentation can be found below.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers

It includes a list of triggers and their available variables. It will not describe specifically each triggers variables in detail as no such documentation exists at this time. You'd need to simply provide what you're wanting to do and we can try to suggest the best trigger for your usage.

That should already be automated. Ensure both CB Activity and CB Auto Actions and installed and up to date. Once done within CB Auto Actions > System Actions publish "CB Articles - Create". Note you need to also have core plugin CB Articles published as its API is used for the activity output. This should cause activity posts to be created automatically when you create a Joomla article. You can make a copy of that system action if you want to customize it further, but most of its output is entirely template driven in CB Activity. The text should be shortened as well as it should only show the text before the readmore break if there is one and it should have a "Read More" expander if the text is too long.

Another option is to simply use the Links feature in CB Activity and link to the article. The Links feature will use a pages metadata for description, title, and even image if available.

It's in your screenshot at the bottom as "CB Articles - Create". You do not need to create this yourself as it's included in CB Auto Actions. Simply publish it within CB Auto Actions > System Actions. It displaying images depends on how you've created the article. If the images are directly in the article they will display fine. If they're under the "Images and Links" tab they won't display as that's custom behavior for the content component that we don't replicate in the activity (but likely will in a future release). Below is what the article activity will look like.