* Add FilterRetriever draft
* Implement FilterRetriever and add tests
* Update comparison to compare whole docs instead of just contents
* Expose FilterRetriever at the retrievers level
* Update docstring (add example usage)
* Add filter_retriever in the API reference docs config
Update retriever search path to start one dir level higher
* simplify _documents_equal
* improve usage example
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Co-authored-by: anakin87 <stefanofiorucci@gmail.com>