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Add read-flavor script and documentation #6245

Job	Run time
docs / Build linux64	1m 7s
lint / lint	20s
test / Linux gcc-11 / Build linux64	1m 38s
test / Linux gcc-12 / Build linux64	1m 46s
test / Windows MSVC / Build win64	3m 59s
test / Run Linux test suite	0s
test / Run Linux test suite (gcc-12, all plugins)	0s
test / Run Windows test suite	0s
test / Run Windows test suite (empty config)	-1s
	8m 49s

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