add check to ensure ToC in README.md matches headings in the file (#541)

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.

.github/workflows/ci.yml

@@ -70,3 +70,5 @@ jobs:
 
       - name: Ensure README.md contains only ASCII and certain Unicode code points
         run: ./scripts/asciicheck.py README.md
+      - name: Check README ToC
+        run: python3 scripts/readme_toc.py README.md