This introduces a Python script (written by Codex!) to verify that the
table of contents in the root `README.md` matches the headings. Like
`scripts/asciicheck.py` in https://github.com/openai/codex/pull/513, it
reports differences by default (and exits non-zero if there are any) and
also has a `--fix` option to synchronize the ToC with the headings.
This will be enforced by CI and the changes to `README.md` in this PR
were generated by the script, so you can see that our ToC was missing
some entries prior to this PR.
Michael Bolin