Document WithApproximate LINQ operator for SQL Server vector search

[roji]

The VectorSearch() API in EF Core has been updated (dotnet/efcore#38144) to replace the topN parameter with a composable LINQ pattern using Take() and a new WithApproximate() operator. This PR updates the documentation to reflect the new API surface.

Key changes:

• SQL Server vector search page: Restructured the "Approximate search with VECTOR_SEARCH()" section into "Searching with VECTOR_SEARCH()" to reflect that the TVF now supports both exact kNN and approximate ANN search. Added a dedicated WithApproximate() subsection explaining its usage and constraints. Updated all code samples and generated SQL to show the new OrderBy().Take().WithApproximate() pattern (replacing the old topN parameter). Updated the hybrid search example and its SQL accordingly.
• What's new page: Updated the VECTOR_SEARCH() section to show the new API with Take() and WithApproximate(), and explain the difference between exact and approximate search modes.