Add instructions to configure SSL client certificates

[ridomin]

Related to #1936

I configured listener.ssl.require_certificate = on and seems that this disables username/password

The docs should clarify if it's really "required", and if is it possible to keep both authentication mechanisms. May I assume without that setting client certs will be accepted? and if so, do I need to add that credential to the users file?

Finally, will be good to have instructions to troubleshoot these issues (I'm new to Verne and not sure how enable/read logs)