Add syntax highlighting to MarkDown code blocks #1846

Job	Run time
specs (ubuntu, 3.0.6)	1m 37s
RuboCop	30s
specs (ubuntu, 3.1.4)	1m 38s
specs (ubuntu, 3.2.2)	1m 29s
specs (ubuntu, 3.3.0)	1m 30s
specs (macos, 3.0.6)	1m 19s
specs (macos, 3.1.4)	1m 36s
specs (macos, 3.2.2)	1m 26s
specs (macos, 3.3.0)	1m 9s
specs (windows, 3.0.6)	4m 26s
specs (windows, 3.1.4)	4m 8s
specs (windows, 3.2.2)	3m 54s
specs (windows, 3.3.0)	5m 3s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 32s
	31m 17s

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