The navigation.json is now passed through the dgeni pipeline.
The source file has been moved to `aio/content/navigation.json`
but the generated file will now appear where the original source file
was found, `aio/src/content/navigation.json`.
Everything inside `aio/src/content` is now generated and ignored by git.
The `processNavigationMap` processor in this commit adds the current version
information to the navigation.json file and verifies the relative urls in
the file map to real documents.
The navigationService exposes the versionInfo as an observable, which the
AppComponent renders at the top of the sidenav.
Peter Bacon Darwin