Option to also exclude navigation entries

[Griperis]

When inheriting from some base template for e.g. internal and external documentation, it's desirable to exclude internal contents from the external documentation, but keep one nav definition in the base template, so there don't have to be duplicate navigation definitions.

We use exclude to exclude the files, but the navigation headers still remain. This PR introduces a new exclude-nav option (opt-in) that also entries from navigation if they reference any of the specified filepaths.