feat(embeddings): add search threshold to BasicEmbeddingsIndex

[Pouyanpi]

This commit introduces a search threshold to the BasicEmbeddingsIndex class. This threshold is used to filter the search results based on their distances. The _filter_results method is added to perform this filtering. The search method is updated to use this new feature. The search_threshold parameter is also added to the LLMRails classs.

TODO:

• [ ] Check if the updated doc is ok