Why are these changes needed?

The contribution guide is split across two sources which don't link to each other: https://github.com/ray-project/ray/blob/master/CONTRIBUTING.rst and https://docs.ray.io/en/latest/getting-involved.html. For example, setup_hooks.sh is only mentioned in the first one, but pip install -r python/requirements_linters.txt is only mentioned in the second one. As a result, most first time contributors need their hand to be held through the process of getting their first PR merged.

This PR establishes https://docs.ray.io/en/latest/getting-involved.html as the source of truth, linking to it from CONTRIBUTING.rst. Ideally a first-time contributor will be able to read this page and independently produce a mergeable PR.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(