docs: add docs for DuckDB extension

[prrao87]

Adds docs showing how to use the new Lance-DuckDB community extension (will need updates based on new updates by @Xuanwo in the coming days).

[github-actions]

ACTION NEEDED
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[github-actions]

Documentation Review

This PR adds comprehensive documentation for the new Lance-DuckDB community extension, replacing the older Arrow-based integration docs.

P1 Issues

1. Missing newline at end of file: The diff shows the file ends without a trailing newline (indicated by \ No newline at end of file). Please add a trailing newline to follow standard file formatting conventions.

2. FTS example may fail without an index: The full-text search example uses lance_search with text_column, but BM25 FTS typically requires an index to be created first. The docs should clarify if an FTS index needs to be created before running FTS queries, or mention that the extension handles this automatically. Without this clarification, users may get errors.

3. Hybrid search positional arguments: In the hybrid search example, the text query and text column are passed as positional arguments after vector_column = 'vector':

   vector_column = 'vector',
   'the duck surprised the dragon',
   'animal',

   Mixing named and positional arguments like this may be confusing and potentially error-prone. Consider using named arguments consistently (e.g., text_query = '...', text_column = 'animal') if the extension supports it.

Minor Suggestions

• The PR description mentions updates are coming from @Xuanwo. Consider waiting for those updates to avoid a second documentation PR, or mark specific sections as subject to change.

Overall the documentation is well-structured with clear examples. The issues above should be addressed to prevent user confusion.

[Xuanwo]

Search APIs have been changed a lot, need a update.

[Xuanwo]

Let's go!

[github-actions]

PR Review

This documentation update for the new DuckDB extension looks well-written with clear examples. A few minor suggestions:

P1 Issues

1. Inconsistent function naming: The introduction mentions lance_search(...) as the unified entry point, but the examples show lance_vector_search(), lance_fts(), and lance_hybrid_search() as separate functions. This could confuse readers. Please clarify if lance_search is actually used or if it should say "The extension exposes several search functions".

Suggestions (non-blocking)

• The FTS example shows searching for "the brave knight faced the dragon" against a single-word column containing only "duck", "horse", and "dragon". This works but may give readers unrealistic expectations about FTS capabilities since only the word "dragon" will match. Consider noting this is a simplified example or using a column with more text.

Overall, this is a solid documentation addition that will help users integrate Lance with DuckDB.