docs: add code fences to `Example:` docstring blocks #822

Job	Run time
checks / pre-commit	42s
checks / readme-snippets	13s
checks / test (3.11, lowest-direct, windows-latest)	2m 38s
checks / test (3.10, locked, ubuntu-latest)	55s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.11, lowest-direct, ubuntu-latest)	58s
checks / test (3.10, lowest-direct, windows-latest)	2m 28s
checks / test (3.11, locked, windows-latest)	2m 22s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 22s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.12, lowest-direct, windows-latest)	3m 34s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.13, locked, windows-latest)	2m 16s
checks / test (3.14, lowest-direct, windows-latest)	2m 20s
checks / test (3.10, locked, windows-latest)	2m 17s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.13, lowest-direct, ubuntu-latest)	51s
checks / test (3.14, locked, windows-latest)	3m 25s
checks / test (3.13, lowest-direct, windows-latest)	2m 46s
checks / test (3.14, locked, ubuntu-latest)	2m 29s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.12, locked, windows-latest)	3m 3s
all-green	4s
	39m 7s

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