roundcube

Add link to version determination instructions page

There needs to be instructions enabling even the least end user to be able to figure out the version number of Roundcube that they are using.

No, not just for administrators, but even the mere end user.

And, not just for when logged in, but even when having problems logging in.

For instance when arguing with a website that they are not using a supported version anymore.

Out of date info

https://github.com/roundcube/roundcubemail/issues/9553 says the info shown is out of date.