markdown: explicitly forbid setext headings

[ee7]

Exercism's markdown spec talks only about ATX headings, but doesn't quite explicitly say "don't use setext headings" https://github.com/exercism/docs/blob/d334042e8f54ba7dd24ad215648355ff7795359a/building/markdown/markdown.md#L9-L15

It does however say:

MD003: Use atx style

So markdown linting in the future will enforce this.

Links:

• https://spec.commonmark.org/0.30/#atx-headings
• https://spec.commonmark.org/0.30/#setext-headings