docs: document log store configuration via [log_store]

[mikeua]

docs/log-store.md and the "Log Store Configuration" section of docs/environment-variables.md describe --log-store-* CLI flags and GRAPH_LOG_STORE_* environment variables (plus "deprecated" GRAPH_ELASTICSEARCH_* aliases). None of these exist: node/src/opt.rs has no such flags, nothing reads those variables, and per #6278 the log store is configured only through the [log_store] section of the TOML config (LogStoreSection in node/src/config.rs). Someone following the guide today gets no log store and no error.

Changes:

• Replace every backend's CLI/env-var examples with [log_store] TOML examples.
• Replace the "Environment Variables / CLI Arguments / Configuration Precedence" section with a table of the actual [log_store] keys, their backends and defaults (index = "subgraph", timeout_secs = 10, retention_hours = 0), and a short migration note for the removed --elasticsearch-* flags / ELASTICSEARCH_* variables.
• Drop the nonexistent file MAX_SIZE / RETENTION_DAYS options; the file backend only has retention_hours.
• Document Loki username/password, which the code supports but the guide didn't mention.
• Since graphql, graph: Error on _logs query when log store disabled #6637, _logs returns a NotSupported error when no store is configured, not empty results; update the two places in the guide that said otherwise, and the matching startup info! message in node/src/launcher.rs.
• Update the troubleshooting entries that pointed at the nonexistent variables.

Docs-only apart from the one log message string.