docs(telemetry): add dashboard-level descriptions for the info button

Consensus Health, Ledger Operations, and Transaction Overview had no
top-level description, so Grafana showed no info button. Add a one-line
summary of what each dashboard shows and its data source.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/telemetry/grafana/dashboards/consensus-health.json

@@ -1142,6 +1142,7 @@
     "from": "now-1h",
     "to": "now"
   },
+  "description": "Consensus round health: round duration, proposal and validation rates, close-time agreement, and ledger-close cadence.",
   "title": "Consensus Health",
   "uid": "xrpld-consensus"
 }

docker/telemetry/grafana/dashboards/ledger-operations.json

@@ -408,6 +408,7 @@
     "from": "now-1h",
     "to": "now"
   },
+  "description": "Ledger lifecycle: build rate and duration, validation rate, and close timing.",
   "title": "Ledger Operations",
   "uid": "xrpld-ledger-ops"
 }

docker/telemetry/grafana/dashboards/transaction-overview.json

@@ -1029,6 +1029,7 @@
     "from": "now-1h",
     "to": "now"
   },
+  "description": "Transaction pipeline overview: processing and receive rates, per-type latency, path distribution, apply-stage breakdown, and validation outcomes.",
   "title": "Transaction Overview",
   "uid": "xrpld-transactions"
 }