Document AuthenticationClass

[fhennig]

currently in the commons operator, but should be moved to the top level docs: https://docs.stackable.tech/commons-operator/stable/authenticationclass.html

This would include

• A concepts page explaining how they work and why they are useful -> Add concepts page on AuthenticationClass and TLS server verification #323
• All CRD attributes are documented (in Rustdoc), so that we can generate reference docs after Spike: generating a CRD reference #238 is implemented

[fhennig]

This is done now