How do I export an entity? In Drupal 7, with the aid of features, we could do something like node_export, or uuid_features, etc.

I'm looking for a good way to create content / other entities using the UI and export it to code some how using Drupal 8 so that I can put into an install or update hook so that our site builders don't miss pieces of configuration.

I know we can do it programatically. And I'm all for that. I would just like an easy way to generate the Array we need to stick in our install or update hooks.
You want the Default Content for D8 module, which provides drush commands to export and installs the content on module install. From its project page:

Default content gives your module and install profile a way to ship default content as well as configuration. Powered by core's serialization, rest and hal modules.
Features

Supports entity-references between content.
Supports files if you have File entity
Easily export your content and its dependencies to yml using drush commands.