docs: document Rust blob v2 read APIs for multimodal tables

[mintlify]

Summary

Adds a new "Blob v2 columns (Rust)" section to the multimodal tables guide so Rust users can discover and use the new blob declaration helper and the read-side materialization APIs.

Changes

• Document the lancedb::blob field helper for declaring blob v2 columns (top-level or nested).
• Document Table::fetch_blobs for eager byte materialization keyed by _rowid.
• Document Table::fetch_blob_files for lazy BlobFile handles when only a subset of payloads is needed.
• Mention Table::blob_columns for schema introspection and call out the Lance 2.2 + stable row ID requirement and the remote-table NotSupported behavior.
• Add three Rust snippet exports in snippets/multimodal.mdx covering schema declaration, query-then-fetch, and lazy handle reads.

Context

Upstream PR shipped the read path for blob v2 columns in the Rust crate, complementing the existing declare + write flow.

[mintlify]

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Project	Status	Preview	Updated (UTC)
lancedb-bcbb4faf	🟢 Ready	View Preview	Jun 23, 2026, 2:23 PM

[prrao87]

Will revisit this once more SDK features are added on top .

[geruh]

Does mintlify reopen these on it's own