When set to visible, the category always appears in the selector
even when no variant exists for the current page. Navigating to
such a page shows pagenotfoundmessage with no fallback to
default-category content. Default behaviour (hidden) is unchanged.
Updates pageShouldDisplay and visibleCategoryCodesForCurrentPage
to honour the new key alongside the existing notfoundmessage logic.
Docs updated with key description, summary table, and full example.
Documents all keys that can appear under default-category and
categories entries: message, name-latin, notfoundmessage,
pagenotfoundmessage, font, and line-height. Adds a summary table
and updates the full example to show these keys in context.
