Enhanced document chunk handling and API

- Updated `Scripts.sql` to add a new `[Index]` column to `[dbo].[DocumentChunks]` for order tracking.
- Modified `DocumentChunk.cs` to include a new `Index` property, and introduced a new immutable record class for document chunks.
- Introduced new API endpoints in `Program.cs` for document and chunk retrieval, including embedding details, with OpenAPI documentation enhancements.
- Updated an API endpoint description in `Program.cs` to clarify document embedding handling.
- Updated `VectorSearchService.cs` to reflect schema changes in service logic, adding methods for fetching document chunks and specific embeddings.

Scripts.sql

@@ -1,6 +1,7 @@
 CREATE TABLE [dbo].[DocumentChunks](
 	[Id] [uniqueidentifier] NOT NULL,
 	[DocumentId] [uniqueidentifier] NOT NULL,
+	[Index] INT NOT NULL,
 	[Content] [nvarchar](max) NOT NULL,
 	[Embedding] [varbinary](8000) NOT NULL,
  CONSTRAINT [PK_DocumentChunks] PRIMARY KEY CLUSTERED