Remove phpcpd (deprecated) from dist and explain docs #636

Job	Run time
Code coverage (codecov)	0s
CI test (make validate)	1m 1s
SelfUpdate tests (PHAR) (8.3)	45s
SelfUpdate tests (PHAR) (8.2)	24s
SelfUpdate tests (PHAR) (8.1)	37s
SelfUpdate tests (PHAR) (8.0)	23s
SelfUpdate tests (PHAR) (7.4)	8s
Build moodle-plugin-ci.phar	27s
Integration tests (8.3, main)	6m 41s
Integration tests (8.2, MOODLE_403_STABLE)	5m 59s
Integration tests (8.2, MOODLE_402_STABLE)	5m 48s
Integration tests (8.1, MOODLE_401_STABLE)	6m 38s
Integration tests (8.0, MOODLE_400_STABLE)	6m 21s
Integration tests (8.0, MOODLE_311_STABLE)	6m 8s
Integration tests (7.4, MOODLE_39_STABLE)	5m 43s
Integration tests (PHAR) (8.3, main)	6m 0s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE)	6m 0s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE)	5m 17s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE)	6m 18s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	6m 26s
	1h 17m 4s

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