docs(authz-keycloak): refresh Keycloak authorization guide

[kayx23]

Description

The authz-keycloak guide still used Keycloak 18.0.2, legacy bootstrap environment variables, screenshots from an older Admin Console, and a runnable Resource Owner Password Credentials example. This PR refreshes the Apache APISIX reference for the current Keycloak Authorization Services workflow.

Changes include:

• synchronize the English and Chinese plugin references
• update the walkthrough and screenshots for Keycloak 26.7.3
• rebuild the Keycloak Authorization Services setup around a service account, resource, scope, client-scope policy, and scope-based permission
• retain dynamic path authorization and static permission examples across the Admin API, ADC, Gateway API, and APISIX CRDs
• prevent the validated bearer token from reaching the sample upstream
• replace the runnable Password Credentials tutorial with compatibility guidance and the RFC 9700 prohibition
• clarify endpoint dependencies, policy-enforcement behavior, redirects, timeout units, and encrypted fields

Validation:

• tested APISIX 3.18.0 with Keycloak 26.7.3 using the current quickstart
• verified dynamic and static configurations through the Admin API: allowed 200, insufficient scope 403, and missing token 401
• ran adc lint and adc sync for both dynamic and static configurations and repeated the allowed/denied requests
• verified that the sample upstream does not receive the Authorization header
• parsed all YAML and JSON examples, verified that English and Chinese code blocks are identical, and ran Markdownlint for both pages
• source-validated the Gateway API and APISIX CRD manifests; no Kubernetes cluster was available for a runtime application test

Which issue(s) this PR fixes:

N/A.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change (documentation-only change; commands were tested manually as described above)
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible