Add TOC to CONTRIBUTING.md (#29671)
Summary:
This TOC is manually generated but `CONTRIBUTING.md` seems like its
stable enough for that to be okay
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29671
Pulled By: driazati
Differential Revision: D18771604
fbshipit-source-id: 0d6c9c6cf1083d3be413219d3cead79c2fe5050b
