Pages are created in the Page Composer. Pages are either Published or in Draft, they are hierarchical and maximum two levels deep. Only published pages are visible to the public, if a parent page is a draft then none of its children will be visible to the public either.
The Page Composer uses a Markdown editor. When the editor is in focus, type Shift + Alt + H
will bring up a Help Dialog that shows you its keyboard shortcuts. To add images just drag & drop them in.
Theme designers can create different layouts for pages to suit different scenarios. The default theme Clarity has 3 layouts.
Parent pages can be asigned to the Site Navigation, parent pages with children will also have Page Navigation.
A parent page with at least one child will have the ability to create a page navigation, like the one shown on the sidebar of the docs . Just make sure you have the Page Navigation widget placed in the according sidebar area.