Navbar

Real landmarks — a header with a nav inside, and a sidebar that is its own named region.

A `<header>` with a `<nav>` in it, not a div with links. Landmarks are what
somebody navigating with a screen reader jumps between, and a page with none
is a page they have to read from the top every time.

`Sidebar` is a separate named landmark for the same reason, and
`Sidebar.Item` marks the current page with `aria-current`.