Docs: Explain how to conditionally hide pages

[DistantOrbit]

Added a paragraph about how to use shouldRegisterNavigation() in page classes to docs.

[danharrin]

Thanks :)

[robertorinaldi-dev]

@danharrin what are the cons in directly making a method that accepts a closure and takes care of the rest automatically? Maybe canView() like for widgets?

[danharrin]

The cons are that every page has different mount() parameters, so if we add mount() to the base Page class we effectively stop people from using mount parameters.