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"

docs/building/markdown/markdown.md

Lines 9 to 15 in d334042

	## Headings
	- All files must start start with a level-1 heading (`# Some heading text`).
	- Level-1 headings exist purely for consuming on GitHub or equivalent.
	- If the file is rendered by Exercism (e.g. displayed on the website, rendered via the CLI), this heading will be removed, and a contextual heading will be inserted.
	- No heading may descend a level greater than one below the previous (e.g. `##` may only be followed by `###`, not `####`).
	- Beyond the single level-1 (`#`) heading, only level-2 (`##`), level-3 (`###`) and level-4 (`####`) headings may be used.

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