KKP: Missing Documentation for Alternative Authentication Provider Under Securing System Services

[Alternative Authentication Provider](https://docs.kubermatic.com/kubermatic/v2.23/architecture/concept/kkp-concepts/kkp-security/securing-system-services/#alternative-authentication-provider) documentation section has reference to the OIDC provider for alternative authentication provider configuration, but the information is in general missing for KKP and IAP integration with alternative authentication provider.

Rather than giving reference to the OIDC Provider configuration, it would be nice if we can elaborate it under the [Alternative Authentication Provider](https://docs.kubermatic.com/kubermatic/v2.23/architecture/concept/kkp-concepts/kkp-security/securing-system-services/#alternative-authentication-provider) itself.

Start with the Alternative Authentication Provider section in the linked Kubermatic documentation and compare it with the referenced OIDC provider configuration. Document the missing KKP and IAP integration details directly in that section, replacing the insufficient reference so both integrations are explained there.

Written by the indexing model from the issue text.