[DOC] Add syntax highlighting to MarkDown code blocks #1430

Job	Run time
steep-check (head)	15s
test-memory (head)	46s
check-misc (head)	11s
test-ruby (head, master)	6m 6s
test-windows (head)	1m 38s
test-ruby (3.0, master)	5m 59s
ruby-versions / Generate Ruby versions	1s
test-cpp (head, g++)	10s
test (2.5)	26s
test (2.6)	28s
test (2.7)	25s
test-cpp (head, clang++)	16s
test (3.0)	16s
test (3.1)	14s
test (3.2)	13s
test (3.3)	14s
test (head)	10s
	17m 48s

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