docs: update explanation for separate config use in litellm (#958)

* docs: update explanation for separate config use in litellm

* docs: update default backend to `rdagent.oai.backend.LiteLLMAPIBackend`

* docs: update .rst format

* Update installation_and_configuration.rst

test/oai/test_embedding_and_similarity.py

@@ -24,6 +24,7 @@ class TestEmbedding(unittest.TestCase):
         assert similarity is not None
         assert isinstance(similarity, float)
         min_similarity_threshold = 0.8
+        print(f"similarity: {similarity}")
         assert similarity >= min_similarity_threshold