Document `--output=json` CLI option (#17611) #48

Job	Run time
Test suite with py38-ubuntu, mypyc-compiled	18m 29s
Test suite with py38-windows-64	20m 16s
Test mypyc suite with 32-bit Python	7m 19s
Test suite with py39-ubuntu	18m 24s
Test suite with py310-ubuntu	16m 18s
Test suite with py311-ubuntu, mypyc-compiled	16m 46s
Test suite with py312-ubuntu, mypyc-compiled	18m 16s
Test suit with py313-dev-ubuntu, mypyc-compiled	20m 5s
mypyc runtime tests with py39-macos	6m 3s
mypyc runtime tests with py38-debug-build-ubuntu	3m 57s
Type check our own code (py38-ubuntu)	1m 48s
Type check our own code (py38-windows-64)	2m 29s
Formatting and code style with Black + ruff	33s
	2h 30m 43s

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