Bug 50674: Allow display of example navbars.

If a user sets $wgBookManagerv2ExampleNavigation to true,
a navigation bar using the example JSON file in
examples/book.json will be added to each mainspace page
that doesn't already have a BookManagerv2 navbar.

Bug: 50674
Change-Id: I6cf0ebc0b6b56e7c4af8a1eba12708af516fc404
Merge "Bug 50674: Allow display of example navbars."