docs: Integration with Sagas for gRPC (follow-up to #3386)

[jeremydmiller]

Follow-up to #3386: adapts the HTTP + Sagas documentation content for gRPC.

What's here

• New docs/guide/grpc/sagas.md page, added to the sidebar and the gRPC section index.
• All samples are mdsnippets regions sourced from real, tested code — the SagaOverGrpc suite merged in gRPC saga coverage: message-identified sagas work like HTTP; header-identified gap pinned (refs #3385) #3386 (ReservationSaga, contracts, service shim, and the end-to-end start → persist → continue → complete test).
• Covers:
  • how the gRPC saga story differs from HTTP (no return-value Saga creation — the shim forwards to Bus.InvokeAsync<T>, so the saga starts via its Start method with message-carried identity);
  • the saga identity conventions on the request DTOs;
  • the header-identified limitation (gRPC service path doesn't propagate saga-id — header-identified sagas can't be started/continued over gRPC #3385) as an explicit callout, with the put-the-id-on-the-message guidance;
  • cascading messages / timeouts / MarkCompleted() working unchanged over the hop.
• Snippet-region annotations added to the four SagaOverGrpc test files (comment tweaks only — no behavior change; the suite still passes 2/2 locally, build clean).

Note: mdsnippets also wanted to refresh line-number links in ~58 unrelated pages from today's merges; those were deliberately left out to keep this diff reviewable — they'll regenerate on the next docs build.

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