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>

docs/test/md_testing.py

@@ -17,6 +17,7 @@ excluded_globs = [
     "../src/basic.md",
     "../src/hybrid_search/hybrid_search.md",
     "../src/reranking/*.md",
+    "../src/guides/tuning_retrievers/*.md",
 ]
 
 python_prefix = "py"