add LCEL to retriever doc (#11888) #7117

Job	Run time
Python 3.8 extended tests	3m 58s
Python 3.9 extended tests	4m 30s
Python 3.10 extended tests	4m 1s
Python 3.11 extended tests	4m 10s
test / Python 3.8	41s
test / Python 3.9	34s
test / Python 3.10	35s
test / Python 3.11	38s
lint / build (3.8)	1m 4s
lint / build (3.11)	49s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.8	38s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.9	41s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.10	40s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.11	53s
	23m 52s

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