Add `DocJson` profile for benchmarking JSON rustdoc output #4239

Job	Run time
Test on Windows	3m 19s
Test and deploy	3m 30s
Test benchmarks (--exclude cargo-0.87.1,stm32f4-0.15.1, Opt)	10m 34s
Test benchmarks (--exclude cargo-0.87.1,stm32f4-0.15.1, Check,Debug,Doc)	11m 44s
Test benchmarks (--include cargo-0.87.1,stm32f4-0.15.1, Check,Debug,Doc)	6m 0s
Test benchmarks (--include cargo-0.87.1,stm32f4-0.15.1, Opt)	7m 40s
Test profiling	4m 32s
Test codegen backend benchmarks	1m 15s
Test runtime benchmarks	4m 35s
Compare page endpoint test	4m 19s
Database Check	1m 11s
	58m 39s

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