* [v3] Add level one headings to Markdown files.

All Markdown files should, from this point on, start with a level one heading.
See https://github.com/exercism/configlet/issues/150 for the rationale behind these changes.

The full specification can be found [here](https://github.com/exercism/docs/blob/main/contributing/standards/markdown.md).

* [v3] Markdown sub-headings must be parent level incremented by one.

The full specification can be found [here](https://github.com/exercism/docs/blob/main/contributing/standards/markdown.md).