Updated CONTRIBUTING.md to focus on repository-specific guidelines #2931

Job	Run time
check-labels / check	2s
check-diff / check	17s
cli / test-cli (ubuntu-latest)	3m 7s
lsp / test-lsp (ubuntu-latest)	6m 41s
building / build-toolchain (ubuntu-latest)	1m 27s
building / epoch / build-epoch	6m 4s
	17m 38s

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