Add "Pages" navigation to Side Menu

I have the pages side panel functional. It's based on the PageInfo component with some design assumptions. I've added a `PageList` component with associated stories to describe the various states, and then updated the `Pages` component to incorporate the `PageList`.

Update the PageList component to use the SidebarNode component to maintain consistency with other sidebars. Updated the layout of the New button to carry over the design and layout of the PageInfo` component.

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