Suggest inclusion of xref on tutorial-tls to howto-tls

Long story short, I was setting up public connectivity with external DNS and followed the links therein to
https://docs.couchbase.com/operator/2.0/tutorial-tls.html

That one leaves you a bit lost at the end with where to put the certs you've created. It turns out this is documented, but there is no reference to:

https://docs.couchbase.com/operator/2.0/howto-tls.html

Recommend adding that reference.