Document the gRPC client for the TS SDK

Follow up of #334. Once `@iota/grpc` lands we need docs for it in this repo, and then whatever is needed in the `iotaledger/iota` monorepo so it shows up on https://docs.iota.org.

## In this repo

- [ ] `sdk/grpc/README.md`: what the package is, install, quick setup, the server-streaming reassembly behaviour and why it matters, the `max_message_size_bytes` / decode limit config, and a note that v1 is Node/Bun/Deno only (no browser, see #334 for the why).
- [ ] TSDoc on the public API so typedoc output is not empty: client constructor + options, the `GetObjects` / `GetTransactions` batch methods, `GetCheckpoint` / `StreamCheckpoints`, and the `List*` cursor paging methods (document that a short page is not the end of the collection).
- [ ] `sdk/grpc/typedoc.json` following the other packages.
- [ ] Add `grpc` to the `packages` list in `scripts/generate-sdk-typedoc.sh` so it gets into the typedoc tarball uploaded by `.github/workflows/ts_sdk_typedoc_upload.yml`.
- [ ] At least one runnable example in `sdk/examples` (checkpoint streaming is the obvious one).

## In iotaledger/iota

Docs site content lives there, so this part is a PR to that repo:

- [ ] New pages under `docs/content/developer/ts-sdk/grpc/`: `index.mdx` (what it is, when to use gRPC over JSON-RPC / GraphQL, supported runtimes), plus how-tos for reading objects and transactions, and for streaming checkpoints.
- [ ] Add a `@iota/grpc` category to `docs/content/sidebars/ts-sdk.js`, including the `API Reference` autogenerated entry pointing at `developer/ts-sdk/grpc/api` like the other packages do.
- [ ] Add `grpc` to the `packages` list in `docs/site/scripts/download-ts-sdk-typedoc.sh`, otherwise the API reference never gets pulled from the tarball.
- [ ] Sanity check the docs build (`pnpm iota-docs build`) before opening the PR, broken links fail the build.

Order matters: typedoc upload from this repo has to ship first, otherwise the monorepo download script has nothing to copy.

Start with sdk/grpc/README.md, the public API TSDoc, sdk/grpc/typedoc.json, scripts/generate-sdk-typedoc.sh, and sdk/examples; compare the typedoc setup in other packages. Then inspect the listed docs/content files and download script in iotaledger/iota, preserving the typedoc upload-before-download order. Done means the package, API reference, examples, site pages, sidebar, and docs build are all covered, with pnpm iota-docs build passing.

Written by the indexing model from the issue text.