[AI Docs] Update vector DB document with links to OSS vector stores

### Describe the issue or suggestion

Feedback for this article: https://learn.microsoft.com/en-us/dotnet/ai/conceptual/vector-databases

The Vector DB article is focused heavily on Azure solutions, with a link to OSS options that is from CosmosDB docs, and doesn't point at .NET resources for vector stores.

We need to update the main document to include a list of vector stores including the OSS stores, instead of linking out to a summary page without details. Alternately, remove the list of vector stores and point at the [ecosystem document](https://learn.microsoft.com/dotnet/ai/dotnet-ai-ecosystem) and its list of vector stores, and update that page with missing items from the vector-databases article.