Keep Markdown ToCs up to date (#54974)
Summary:
This PR uses [markdown-toc](https://github.com/jonschlinkert/markdown-toc#cli) to [automatically update the table of contents for `README.md` and `CONTRIBUTING.md`](https://github.com/pytorch/pytorch/pull/54904#issuecomment-809682134) in CI.
This keeps the same format already used in `README.md`. While it does slightly change the format for the ToC in `CONTRIBUTING.md`, the new format is actually just the same as the old format that was already being used prior to https://github.com/pytorch/pytorch/issues/51458.
Race condition with https://github.com/pytorch/pytorch/issues/54904.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54974
Test Plan: The new "Lint / toc" job in GitHub Actions [succeeds](https://github.com/pytorch/pytorch/pull/54974/checks?check_run_id=2238739005) on this PR, and [fails](https://github.com/pytorch/pytorch/pull/54976/checks?check_run_id=2238784022) on https://github.com/pytorch/pytorch/issues/54976 with an understandable error message.
Reviewed By: malfet
Differential Revision: D27468390
Pulled By: samestep
fbshipit-source-id: 14a73f42ed546d4310140b94ded14e099185d0e0
