improve `optionals` guide #1685

Job	Run time
Install packages	17s
Run ESLint in library	49s
Run Prettier in library	33s
Build and publish library	33s
Run Vitest in library	37s
Run ESLint in packages/to-json-schema	27s
Run Prettier in packages/to-json-schema	24s
Run Vitest in packages/to-json-schema	19s
Run ESLint in website	28s
Run Prettier in website	50s
	5m 17s

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