Mention TLS server verification in AuthClass OIDC docs

[sbernauer]

Raised in stackabletech/superset-operator#534

Document tls property for an OIDC AuthClass in https://docs.stackable.tech/home/nightly/concepts/authentication#OIDC, ideally linking to https://docs.stackable.tech/home/nightly/concepts/tls-server-verification

[NickLarsenNZ]

Just adding some relevant conversation from a while back when we were discussing splitting hostname and port, vs making it a url (where tls can be implied from): #460 (comment)