add documentation for gssapi authentication

[novicr]

Documentation regarding changes to Kafka scaler to support Kerberos authentication

PR: kedacore/keda#4851
Feature request: kedacore/keda#4836

Checklist

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Fixes #

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[tomkerkhove]

Can you also link the issue + feature PR in the PR description please?

[novicr]

Can you also link the issue + feature PR in the PR description please?

updated

[JorTurFer]

Could you add an example about how to use this new authentication?

[novicr]

Could you add an example about how to use this new authentication?

Added example for using gssapi auth

[zroubalik]

Looking good, just a nit

[zroubalik]

Please move this change to docs/2.13

We shipped 2.12 already.