Warn about non-up-to-date version on docs.nextcloud.com page

[krichter722]

It'd be nice if a visitor of documentation pages at https://docs.nextcloud.com/desktop, e.g. https://docs.nextcloud.com/desktop/2.3/conflicts.html would receive a warning that a more up-to-date version (https://docs.nextcloud.com/desktop/2.5/conflicts.html is available). Even if the up-to-date version might not have changed, it's still more a good idea to switch to the latest version for bookmarking and linking.