feat: improve contributing docs #873

Job	Run time
Compile Benchmarks	1m 11s
Checks	1m 30s
Compile and test (2.12.19, 8)	3m 42s
Docs	2m 26s
Compile and test (2.12.19, 11)	3m 36s
Compile and test (2.12.19, 17)	3m 41s
Gradle tests	3m 4s
Maven tests	2m 36s
Compile and test (2.13.14, 8)	4m 23s
sbt scripted tests (gen-scala-server, 2.12)	7m 51s
Compile and test (2.13.14, 11)	3m 59s
sbt scripted tests (gen-java, 2.12)	3m 15s
Compile and test (2.13.14, 17)	4m 10s
Compile and test (3.3.3, 8)	4m 32s
Compile and test (3.3.3, 11)	4m 12s
Compile and test (3.3.3, 17)	4m 15s
	58m 23s

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