Layouts are a powerful way to reuse common elements like sidebars / navbars across multiple pages. You can also use layouts to create dialogs and drawers.
Adding a layout to your page
Each page can be configured with the following layout options:
No layout
Default layout - for navigational elements
Dialog layout - for modals over existing content
Drawer layout - for right-side drawers with extra details
To add a layout to your page:
Open your page and ensure you have nothing selected.
Click on the layout on the right-hand side under Page Layout
Customizing the default layout
To add a sidebar or navbar to your default layout:
Open any page
Click on the pencil icon in Page Layout > Default on the right-hand side
Click on "Choose layout template"
Select a sidebar / navbar layout
Click on "Apply layout" to make it your default layout
From here, you can continue editing your layout such as changing the text or adding new items.