[chore] Improve check/fix make targets

[joaopgrassi]

Changes

I often run into a problem where I modify a markdown and forget to update the TOC, and then the PR fails. My flow is:

• Make changes
• Run make check
• Fix things, commit, push

But I noticed that make check does not include the TOC target which makes it not so useful.

It's unfortunate, but today, the toc library does not offer a "check" or "dry-run" feature (there's a PR open jonschlinkert/markdown-toc#148), so the only way to check is to execute it, and verify if there's any pending changes using git, which is exactly what we do in CI today.

I propose that we add the same thing to make check so at least I know locally that I'm missing something and need to commit. I also added markdown-toc to fix even though it will already be fixed with make check. I know it isn't great, but at least me personally, prefer this over waiting for a PR to fail and then having to push again.

Note: Also I found out that prettier is formatting the CONTRIBUTING.md with - bullet points, where markdown-toc uses * by default. So I also changed that and that's why all the modified files (sorry)

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[ChrsMark]

LGTM