docs: add inline example plot to sc.pl.pca_variance_ratio #1702

Job	Run time
get-environments	10s
build	25s
test (hatch-test.low-vers, coverage, 3.12)	9m 44s
test (hatch-test.stable, 3.14)	6m 7s
test (hatch-test.pre, coverage, 3.14)	6m 35s
test (hatch-test.few-extras, 3.14)	5m 9s
check	3s
	28m 13s

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