docs: add retriever guide, address minor onboarding feedbacks & enhancement (#1326)

- Tried to address some onboarding feedbacks listed in
https://github.com/lancedb/lancedb/issues/1224
- Improve visibility of pydantic integration and embedding API. (Based
on onboarding feedback - Many ways of ingesting data, defining schema
but not sure what to use in a specific use-case)
- Add a guide that takes users through testing and improving retriever
performance using built-in utilities like hybrid-search and reranking
- Add some benchmarks for the above
- Add missing cohere docs

---------

Co-authored-by: Weston Pace <weston.pace@gmail.com>

python/python/tests/docs/test_embeddings_optional.py

@@ -0,0 +1,27 @@
+import lancedb
+
+# --8<-- [start:imports]
+from lancedb.pydantic import LanceModel, Vector
+from lancedb.embeddings import get_registry
+
+# --8<-- [end:imports]
+import pytest
+
+
+@pytest.mark.slow
+def test_embeddings_openai():
+    # --8<-- [start:openai_embeddings]
+    db = lancedb.connect("/tmp/db")
+    func = get_registry().get("openai").create(name="text-embedding-ada-002")
+
+    class Words(LanceModel):
+        text: str = func.SourceField()
+        vector: Vector(func.ndims()) = func.VectorField()
+
+    table = db.create_table("words", schema=Words, mode="overwrite")
+    table.add([{"text": "hello world"}, {"text": "goodbye world"}])
+
+    query = "greetings"
+    actual = table.search(query).limit(1).to_pydantic(Words)[0]
+    print(actual.text)
+    # --8<-- [end:openai_embeddings]