docs: document nested vector column discovery for search

[mintlify]

Summary

Document the new behavior where LanceDB automatically discovers a single vector column nested inside a struct when running searches or creating an index.

Changes

• Added a "Selecting the vector column" section to the Vector Search guide that explains automatic discovery, the dot-notation field path (with backtick escaping), how dimension matching disambiguates multiple candidates, and what the error messages mean.
• Linked from the Vector Index page so the index-creation example points readers at the same syntax.

Context

Follows the upstream change that recurses into struct fields when resolving the default vector column, so users with a single nested vector leaf no longer need to pass an explicit field path.

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