Add PKCE specific Keycloak setup

[javorszky]

Proposed changes

As part of the work for nginx/kubernetes-ingress#7765, we need to update the documentation here on how to set up keycloak with pkce as the nic docs link to the changed document.

There is no issue on the documentation repository

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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
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[javorszky]

I also updated setting up keycloak to the current version of it (25.0.2)

[javorszky]

I have hereby read the F5 CLA and agree to its terms

[javorszky]

recheck