Add AKS External Identity Provider with self-hosted Keycloak to MS Docs

**Describe your scenario**
The AKS external identity provider covers GitHub and Google Identity, both SaaS products. Both scenarios are described in MS Docs. There are other scenarios possible and I would love to see self-hosted Keycloak as another scenrios listed as well. This has already been tested in my lab and it works.
This pull request had been raised (also covering a link to my blog post and all steps), however, it was closed with commend to reach out to PM here and let them make this decision.

https://github.com/MicrosoftDocs/azure-aks-docs/pull/377#issuecomment-4077856934

Best regards,
Tobias