Updated CONTRIBUTING.md to focus on repository-specific guidelines #3031

Job	Run time
check-diff / check	17s
serialization / serialization-tests	7m 15s
c / benchmarking / benchmark-tests	6m 44s
c / ccpp / regular-tests (ubuntu-latest)	22m 31s
py / py-tests / regular-tests (ubuntu-latest)	16m 53s
cpp / cpp-benchmark-tests / benchmark-tests	12m 10s
c / default / regular-tests (ubuntu-latest)	26m 8s
c / verifier / run (ubuntu-latest)	5m 46s
cpp / cpp-tests / regular-tests (ubuntu-latest)	48m 46s
cpp / cpp-ros2-tests / cpp-ros2-tests	38m 30s
rs / rs-benchmark-tests / benchmark-tests	16m 0s
rs / rs-tests / regular-tests (ubuntu-latest, stable)	8m 27s
ts / benchmarking / benchmark-tests	3m 49s
ts / ts-tests / regular-tests (ubuntu-latest)	18m 57s
	3h 52m 13s

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