Split the topbar-nav to a separate file for easier overriding

The header-content.hbs file is used to created the top header, including the logo, the search bar, the drop-down menus, and the Download PDF button. Several sites want to override the drop-down menus or remove the Download PDF button, but to do so they have to duplicate the logic for the things they are keeping. This duplication creates a potential problem -- any update to the main UI needs to be replicated to their overrides.

Since the common case is overriding the drop-down menus, split those into a separate file. Then, custom sites can override that while leaving the rest in tact.

This also removes the "Download PDF" button, since that wasn't working and will need to be different once we get Antora Assembler going.