Extension pages should be able to list addins even without addin category

> From @pascalberger's comment on https://github.com/cake-build/website/issues/1156#issuecomment-752982078
>
> the extension page we should be able to list addins even without addin category (but we need addin category for references page).

Locate the extension page templates and the logic that requires an addin category, then compare that behavior with the references page requirement. Make extension pages list addins without requiring the category while retaining the category requirement for references pages; verify both page types render the intended listings.

Written by the indexing model from the issue text.