Document adding embeddings to empty table in Rust SDK

[prrao87]

Till recently, the Rust SDK's create_empty_table() method did not support an embedding registry to add embeddings to an empty table. Even using .add_embedding() in the builder pattern had no effect - the underlying TableDefinition::new_from_schema() marks all columns as physical, erasing embedding metadata. Rust developers could not create tables with embeddings using the documented API, forcing them to find workarounds or switch to Python. See lancedb/lancedb#2759 for more context on the user-reported issue.

The fix for this was recently merged in a recent PR: lancedb/lancedb#2961 The right way to document this would be to write a test in tests/rs in the docs repo, generate a snippet and add that snippet to the embedding/index.mdx file.

[prrao87]

This has been completed in #154.