docs: Add comprehensive Keycloak configuration guide for Canton Valid…

[mikeProDev]

…ator

Adds new community guide 'Complete Keycloak Configuration Guide for Canton Validator' with detailed step-by-step instructions for setting up Keycloak as an OIDC provider for validator node deployments.

The guide covers:

• Realm and session configuration
• Client scopes setup (daml_ledger_api, openid)
• Multiple client configurations (ledger-api, validator-app-backend, wallet-web-ui, cns-ui)
• User management with proper PARTY_HINT username matching
• Environment variable configuration for validator applications
• Validation testing and troubleshooting steps

Pull Request Checklist

Cluster Testing

• If a cluster test is required, comment /cluster_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If a hard-migration test is required (from the latest release), comment /hdm_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.

PR Guidelines

• Include any change that might be observable by our partners or affect their deployment in the release notes.
• Specify fixed issues with Fixes #n, and mention issues worked on using #n
• Include a screenshot for frontend-related PRs - see README or use your favorite screenshot tool

Merge Guidelines

• Make the git commit message look sensible when squash-merging on GitHub (most likely: just copy your PR description).

[martinflorian-da]

Nice, thank you @mikeProDev ! Do you want to add a note about this to the release notes before I click merge? We can also follow up on that after we merge.

[mikeProDev]

No, I don't have anything to add

[martinflorian-da]

Thanks!