Add `Labels` section to CONTRIBUTING.md + update changelog skip labels

[fmoletta]

Adds a Labels section with short descriptions of commonly-used PR labels to CONTRIBUTING.md. Removes coverage and adds tests & documentation labels to changelog workflow's skipLabels list. Resaoning:

• tests & coverage labels are often used for the same purposes so it doesn't make sense to have both. (And having PRs with only improving coverage rather than testing as an objective has lead to unuseful tests being added in the past).
• There is no reason for documentation updates to be reflected in the changelog

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9fb1b83) 97.71% compared to head (160c161) 97.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1620 +/- ## ======================================= Coverage 97.71% 97.71% ======================================= Files 91 91 Lines 37301 37301 ======================================= Hits 36448 36448 Misses 853 853 ```

:umbrella: View full report in Codecov by Sentry.
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[github-actions[bot]]

Benchmark Results for unmodified programs :rocket:

Command	Mean [s]	Min [s]	Max [s]	Relative
base big_factorial	2.318 ± 0.068	2.270	2.502	1.01 ± 0.04
head big_factorial	2.301 ± 0.049	2.270	2.436	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base big_fibonacci	2.290 ± 0.017	2.262	2.317	1.01 ± 0.01
head big_fibonacci	2.272 ± 0.023	2.244	2.304	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base blake2s_integration_benchmark	8.613 ± 0.080	8.529	8.721	1.00
head blake2s_integration_benchmark	8.667 ± 0.206	8.504	9.221	1.01 ± 0.03

Command	Mean [s]	Min [s]	Max [s]	Relative
base compare_arrays_200000	2.394 ± 0.021	2.361	2.427	1.00 ± 0.01
head compare_arrays_200000	2.383 ± 0.016	2.370	2.418	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base dict_integration_benchmark	1.475 ± 0.008	1.461	1.493	1.01 ± 0.01
head dict_integration_benchmark	1.467 ± 0.006	1.459	1.477	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base field_arithmetic_get_square_benchmark	1.398 ± 0.007	1.389	1.411	1.00
head field_arithmetic_get_square_benchmark	1.413 ± 0.042	1.391	1.531	1.01 ± 0.03

Command	Mean [s]	Min [s]	Max [s]	Relative
base integration_builtins	8.545 ± 0.032	8.500	8.601	1.00 ± 0.01
head integration_builtins	8.524 ± 0.040	8.482	8.618	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base keccak_integration_benchmark	8.950 ± 0.334	8.745	9.817	1.01 ± 0.04
head keccak_integration_benchmark	8.841 ± 0.062	8.752	8.916	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base linear_search	2.402 ± 0.038	2.380	2.510	1.01 ± 0.02
head linear_search	2.384 ± 0.025	2.364	2.443	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base math_cmp_and_pow_integration_benchmark	1.852 ± 0.025	1.831	1.918	1.02 ± 0.02
head math_cmp_and_pow_integration_benchmark	1.823 ± 0.014	1.807	1.861	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base math_integration_benchmark	1.635 ± 0.032	1.605	1.725	1.00
head math_integration_benchmark	1.635 ± 0.024	1.608	1.692	1.00 ± 0.02

Command	Mean [s]	Min [s]	Max [s]	Relative
base memory_integration_benchmark	1.297 ± 0.003	1.293	1.301	1.00
head memory_integration_benchmark	1.303 ± 0.022	1.290	1.363	1.00 ± 0.02

Command	Mean [s]	Min [s]	Max [s]	Relative
base operations_with_data_structures_benchmarks	1.922 ± 0.014	1.906	1.948	1.00
head operations_with_data_structures_benchmarks	1.932 ± 0.031	1.905	2.016	1.01 ± 0.02

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base pedersen	601.6 ± 8.4	596.3	624.9	1.01 ± 0.01
head pedersen	597.7 ± 2.8	594.9	604.8	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base poseidon_integration_benchmark	1.012 ± 0.007	1.005	1.029	1.01 ± 0.01
head poseidon_integration_benchmark	1.005 ± 0.005	0.999	1.011	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base secp_integration_benchmark	1.945 ± 0.010	1.936	1.966	1.00
head secp_integration_benchmark	1.957 ± 0.010	1.947	1.980	1.01 ± 0.01

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
base set_integration_benchmark	739.2 ± 6.6	730.2	752.7	1.01 ± 0.01
head set_integration_benchmark	734.8 ± 4.0	729.3	742.3	1.00

Command	Mean [s]	Min [s]	Max [s]	Relative
base uint256_integration_benchmark	4.702 ± 0.027	4.667	4.737	1.00
head uint256_integration_benchmark	4.734 ± 0.125	4.663	5.075	1.01 ± 0.03

[Oppen]

I think only code owners should be applying the labels.