docs: add KCP log rejection debugging guide and dashboard for SAP Cloud Logging

[atisap]

Description

Changes proposed in this pull request (what was done and why):

• Added a new "Debug KCP Log Rejections" section to the SAP Cloud Logging integration guide, explaining how to identify and diagnose log rejections from KCP applications using the cls-rejected-* index and SAP Cloud Logging Dev Tools.
• Added a new Grafana/OpenSearch dashboard definition dashboard-kcp-log-rejections.ndjson that visualizes rejected logs from the logs-json-kyma index prefix, broken down by KCP application (kubernetes.app_name).
• Linked the new dashboard in the "Use SAP Cloud Logging Dashboards" section of the README.

Changes refer to particular issues, PRs or documents:

• SAP Cloud Logging integration guide: docs/user/integration/sap-cloud-logging/README.md

Traceability

• The PR is linked to a GitHub issue.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• If a CRD is changed, the corresponding Busola ConfigMap has been adjusted.
• The feature is unit-tested.
• The feature is e2e-tested.

[atisap]

https://github.tools.sap/kyma/backlog/issues/8197

[a-thaler]

Hi @atisap, this guide is meant for end-users of the telemetry module and is external available, so we would need to remove the "KCP" flavor. Before we just do it, could you explain what is the value compared to the dashboard available in Cloud Logging? Can't we maybe do a feature request to Cloud Logging to have for example the index selectable?