[CONTRIBUTING] Document issue labels and triage process

[dbort]

Summary

Add a brief description of the process that the ExecuTorch oncall follows to triage new issues. Also describe the most common labels.

Test plan

Previewed in github. Clicking on the links goes to the expected locations.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/8043

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure, 1 Pending

As of commit f76dac5 with merge base 8990dc0 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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