Retrievers components: add "Overview" in docs

[dfokina]

The goal is not to explain all the parameters available, but rather the things that users needs to prepare to be able to work with the component (API keys, understanding the data classes, understanding how to choose the parameters, having a provider account or any other other things). Some examples are in the AnswerBuilder docs or MetaFieldRanker.

### Tasks
- [ ] InMemoryBM25Retriever
- [ ] InMemoryEmbeddingRetriever
- [ ] AstraRetriever
- [ ] ChromaEmbeddingRetriever
- [ ] ChromaQueryRetriever
- [ ] ElasticsearchBM25Retriever
- [ ] ElasticsearchEmbeddingRetriever
- [ ] OpenSearchBM25Retriever
- [ ] OpenSearchEmbeddingRetriever
- [ ] PineconeDenseRetriever
- [ ] QdrantRetriever