Layout
You specify the layout for a page in its front matter. Just the Docs has a default layout with a sidebar, used for almost all pages in the theme docs, and a minimal layout that omits the sidebar.
The layout concept
See the Jekyll docs page about layouts for an explanation of the general idea of layouts and how to specify them.
You can use Jekyll’s front matter defaults to specify the same layout for many pages.
The default layout
This page uses the default layout.
It is a responsive layout: on medium and larger width displays, it displays a sidebar, including a navigation panel; on smaller width displays, the sidebar is automatically hidden under a button.
Each child (and grandchild) page of a top-level page has so-called breadcrumbs: links to its parent (and grandparent) pages. It shows the breadcrumbs above the main content of the page.
Each page that has child pages generally has a list of links to those pages (you can suppress it by has_toc: false in the front matter). It shows the list as a table of contents below the main content.
The minimal layout
A child and grandchild page of this page use the minimal layout. This differs from the default layout by omitting the sidebar – and thereby also the navigation panel. To navigate between pages with the minimal layout, you can use the breadcrumbs and the tables of contents.
Other layouts
Just the Docs has further layouts: about, home, page, and post. Currently, they are all based on the default layout. See the Jekyll docs about inheritance for how to customize them.