Add table of contents to README

[lorensr]

I like these, for helping people know what content it has, and easily jumping to a specific thing. While there is a built-in button, many don't know of it, and it's harder to get a sense of everything because it's small and scrolling.

Updatable either manually or with npx doctoc README.md

[cretz]

Hrmm, I like it. My only concern is keeping it up to date. I can't find a --check-only type of option for doctoc. Think instead we can run it in CI and confirm using a git command that README.md didn't change?

I do this with proto gen today. And I have already marked one matrix run as docsTarget. So, including whatever npm setup is needed, you could add:

      # Confirm README ToC is generated properly
      - name: Check generated README ToC
        if: ${{ matrix.docsTarget }}
        run: |
          npx doctoc README.md
          [[ -z $(git status --porcelain README.md) ]] || (git diff README.md; echo "README changed"; exit 1)

(ideally node install would either be same step or at least right near it with same conditional...feel free to push a known failure to confirm the check works before pushing the proper)

[lorensr]

Confirming that the new step fails as expected:

build-lint-test (3.11, ubuntu-latest)

Now fixing so it'll pass

[cretz]

Thanks! Will merge once CI completes.

[cretz]

Fantastic, thanks!