Move missing_doc_code_examples lint behind its feature gate #274

Job	Run time
Check (stable)	12s
Clippy (stable)	15s
Miri	1m 58s
Rustfmt (stable)	7s
Test Suite (stable)	30s
Test Suite (ubuntu-latest)	48s
Test Suite	3m 42s
Test Suite (stable, --no-default-features)	1m 0s
Test Suite (windows-latest)	2m 46s
Test Suite (--no-default-features)	3m 39s
Test Suite (stable, --features track-caller)	1m 6s
Test Suite (macOS-latest)	1m 11s
Test Suite (--features track-caller)	3m 49s
Test Suite (stable, --features pyo3)	57s
Test Suite (--features auto-install)	3m 42s
Test Suite (stable, --features auto-install)	31s
Test Suite (stable, --all-features)	30s
Test Suite (beta)	37s
Test Suite (beta, --no-default-features)	1m 15s
Test Suite (beta, --features track-caller)	34s
Test Suite (beta, --features pyo3)	35s
Test Suite (beta, --features auto-install)	1m 6s
Test Suite (beta, --all-features)	1m 0s
Test Suite (nightly)	44s
Test Suite (nightly, --no-default-features)	1m 11s
Test Suite (nightly, --features track-caller)	45s
Test Suite (nightly, --features pyo3)	1m 24s
Test Suite (nightly, --features auto-install)	45s
Test Suite (nightly, --all-features)	1m 34s
	38m 13s

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