Improve OTEL docs on body format

[jakubdyszkiewicz]

Description

Context kumahq/kuma#8348

A user as well as me got confused how to use body field. We need examples like this

apiVersion: kuma.io/v1alpha1
kind: MeshAccessLog
metadata:
  name: default
  namespace: kuma-system
spec:
  targetRef:
    kind: Mesh
  from:
    - targetRef:
        kind: Mesh
      default:
        backends:
          - type: OpenTelemetry
            openTelemetry:
              endpoint: otel-collector.observability.svc:4317
              body:
                kvlistValue:
                  values:
                  - key: "mesh"
                    value:
                      stringValue: "%KUMA_MESH%"

[bartsmykla]

xref: added example to openapi schema of MeshAccessLog: https://github.com/kumahq/kuma/pull/8533/files#diff-ed8e7e0fddb480be4d601ae35bb953cddd53659fb3b9d306b43996a8f322cb38R82

[github-actions]

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