Summary:
The lint was originally added in https://github.com/pytorch/pytorch/issues/54974, but at the time I didn't realize that these other Markdown files also each have a table of contents:

- `GLOSSARY.md`
- `torch/csrc/jit/OVERVIEW.md`
- `torch/csrc/jit/docs/serialization.md`
- `torch/fx/OVERVIEW.md`

This PR adds those files to the lint, and also changes the rule from using a fixed list of filenames to a `git grep` command that finds all Markdown files containing this magic comment:

```md

```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/56487

Test Plan: The "Lint / toc" job in GitHub Actions.

Reviewed By: janeyx99

Differential Revision: D27884885

Pulled By: samestep

fbshipit-source-id: 5462437502b17fba93abf5098e21754bf566a4fe