docs: add inline example plot to sc.pl.correlation_matrix #1705

Job	Run time
get-environments	8s
build	25s
test (hatch-test.stable, 3.14)	6m 6s
test (hatch-test.few-extras, 3.14)	5m 37s
test (hatch-test.pre, coverage, 3.14)	6m 32s
test (hatch-test.low-vers, coverage, 3.12)	10m 7s
check	2s
	28m 57s

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