Once you publish your first decision, Confluence will add a decision register to the sidebar for the space:
Customize your template
Decision pages are created from a blueprint template, which means they come bundled with Confluence and are available across your whole site. Blueprint templates can be customized for each space, and the site admin can even change the default blueprint template for your whole site. To learn more about customizing blueprint templates or creating your own templates, see Blueprints and user-created templates.