Docs: Improve Keycloak configuration steps

[avm99963]

The latest available Keycloak version (v25) doesn't add the roles to the ID token. Thus Grafana is not able to extract the roles.

This commit improves the docs by adding a step which explains how to include the roles in the ID token.

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Please check that:

• [x] (not applicable) It works as expected from a user's perspective.
• [x] (not applicable) If this is a pre-GA feature, it is behind a feature toggle.
• [x] The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[CLAassistant]

All committers have signed the CLA.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 2 weeks if no further activity occurs. Please feel free to give a status update or ping for review. Thank you for your contributions!

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 2 weeks if no further activity occurs. Please feel free to give a status update or ping for review. Thank you for your contributions!

[github-actions[bot]]

This pull request has been automatically closed because it has not had any further activity in the last 2 weeks. Thank you for your contributions!