Add-ons for Google Docs, Sheets, and Forms are created by outside developers to increase the functionality of these apps. Add-ons can not be found in the Chrome Web Store; rather, you get add-ons while you are in a Google Doc, Sheet, or Form.
How to install Add-ons:
First, you need to have a Google Doc, Sheet, or Form open. Go to the Add-ons menu and choose Get add-ons. Search or browse for an appropriate add-on.