Add doc section to explain how to use DebugTimer for custom profiling #633

Job	Run time
testsuite-linux (7.4, pgsql)	32s
testsuite-linux (7.4, mysql)	36s
testsuite-linux (7.4, sqlite)	35s
testsuite-linux (8.0, pgsql)	57s
Coding Standard & Static Analysis	27s
testsuite-linux (8.2, sqlite)	36s
testsuite-linux (8.0, mysql)	44s
testsuite-linux (7.4, sqlite, prefer-lowest)	43s
testsuite-linux (8.1, sqlite)	26s
testsuite-linux (8.0, sqlite)	28s
	6m 4s

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