[GUIDE] Walkthrough for `b2nd-to-zarr` CLI tool

[FrancescAlted]

b2nd_to_zarr is a robust CLI tool supporting Zarr v2 and v3, sharding (--sharded, --shards), custom chunk/codec mapping, memory buffer control (--buffer-size), and data integrity verification (--verify, --full-verify).

While parquet-to-blosc2 has a dedicated walkthrough guide in doc/guides/parquet_to_blosc2.rst, b2nd-to-zarr currently has no guide in doc/guides/ and is only mentioned in a single line within the installation docs.

Suggested Work

Create a new Markdown guide at doc/guides/b2nd_to_zarr.md (and add it to doc/guides/index.rst under Command line tools).

Cover the following steps with runnable shell and Python snippets:

1. Prerequisites & Installation: Installing the [zarr] optional extra (pip install "blosc2[zarr]").
2. Step 1 — Create a sample .b2nd array: A small Python snippet to generate a sample multidimensional array.
3. Step 2 — Basic export to Zarr v3: Converting the file using b2nd-to-zarr sample.b2nd sample.zarr.
4. Step 3 — Custom chunk layout & memory buffer: Overriding chunk shapes (--chunks) and controlling memory footprint with --buffer-size.
5. Step 4 — Zarr v3 Sharding: Using --sharded to map Blosc2 chunks to Zarr shards and Blosc2 blocks to inner Zarr chunks.
6. Step 5 — Codec and filter selection: Explicitly specifying codecs (--codec zstd, --clevel 5, --shuffle bitshuffle).
7. Step 6 — Integrity verification: Checking data consistency with --verify and --full-verify.
8. Cross-link with doc/guides/parquet_to_blosc2.rst and the Zarr source reference docs.