Add doc validation workflow + fix linting errors #850

Job	Run time
cs-stan / Coding Standard & Static Analysis	31s
testsuite / testsuite-linux (8.2, highest, false)	38s
testsuite / testsuite-linux (8.3, highest, false)	29s
testsuite / testsuite-linux (8.2, lowest, false)	31s
testsuite / testsuite-linux (8.4, highest, false)	1m 17s
testsuite / testsuite-linux (8.5, highest, false)	1m 19s
	4m 45s

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