docs: align log-store docs with [log_store] TOML and pin Rust 1.94.0

[imitater-dou]

Summary

Align log-store documentation with the real [log_store] TOML config, and pin the README Rust prerequisite to the toolchain in rust-toolchain.toml.

Repro (tip 6838f4e3cf36)

1. Fake CLI flags in README / docs

   • README.md Log Storage quick example uses --log-store-backend file / --log-store-file-dir.
   • docs/log-store.md documents a full --log-store-* CLI surface and GRAPH_LOG_STORE_* env vars, plus retention_days / max_size.
   • node/src/opt.rs has no --log-store-* flags (search for log_store / log-store → no matches).
   • Real config is LogStoreSection in node/src/config.rs under [log_store] in the TOML passed with --config (directory, retention_hours, url, …). See also node/resources/tests/full_config.toml.
   • NEWS.md for v0.44.0 (*: Expose subgraph logs via subgraph GraphQL  #6278) states Elasticsearch CLI/ELASTICSEARCH_* were removed in favor of [log_store].

2. Fictional env vars in docs/environment-variables.md

   • Lists GRAPH_LOG_STORE_*. Code search for GRAPH_LOG_STORE hits only docs files, not runtime source.

3. Rust pin mismatch

   • README says install “latest stable” / rustup install stable.
   • rust-toolchain.toml pins channel = "1.94.0" (with a comment about the 1.95.0 regression).

What changed

• README.md: pin Rust to 1.94.0; replace fake --log-store-* example with a [log_store] TOML snippet + --config.
• docs/log-store.md: document TOML-only configuration (field table matching LogStoreSection), fix retention to retention_hours, add migration notes from removed ES CLI (*: Expose subgraph logs via subgraph GraphQL  #6278), update troubleshooting.
• docs/environment-variables.md: remove fictional GRAPH_LOG_STORE_* list; point readers at [log_store] / the Log Store guide.

Docs-only; no runtime changes.