Add doc section to explain how to use DebugTimer for custom profiling #634

Job	Run time
testsuite / testsuite-linux (8.2, pgsql, highest, false)	37s
cs-stan / Coding Standard & Static Analysis	31s
testsuite / testsuite-linux (8.2, mysql, highest, false)	57s
testsuite / testsuite-linux (8.3, sqlite, highest, false)	1m 2s
testsuite / testsuite-linux (8.5, sqlite, highest, true)	19s
testsuite / testsuite-linux (8.2, sqlite, lowest, false)	28s
testsuite / testsuite-linux (8.4, mysql, highest, false)	43s
	4m 37s

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