markdown: explicitly say whether GFM is supported

I believe that the website uses [`commonmarker`](https://github.com/gjtorikian/commonmarker) (see [here](https://github.com/exercism/website/blob/498497e3cd333c248aee5eaa5ee6d2298b0349eb/app/commands/markdown/render_html.rb#L18)), which is the wrapper for [`libcmark-gfm`](https://github.com/github/cmark-gfm). And therefore the website supports rendering a markdown file that uses [GitHub Flavored Markdown](https://github.github.com/gfm/) features.

However, the [Exercism Markdown spec](https://github.com/exercism/docs/blob/d3e8cb88eef0ade7401d95e349f469a3a3a8faf3/building/markdown/markdown.md) does not state whether a Markdown file can use GFM features. We should state that explicitly, so that e.g. a track knows whether they can use a tables, task list items, or strikethroughs.

Configlet will soon learn to parse and render Markdown, but this will at least initially only support the CommonMark 0.30 spec (not GFM). If necessary, it should be straightforward for it to support GFM (by wrapping `libcmark-gfm`, rather than `libcmark`).

Somewhat related:
GitHub is thinking about adding a new admonition syntax. See https://talk.commonmark.org/t/4173

But even if the GFM spec is changed, I would suggest that we keep the current admonition syntax:

> All blocks are written using 4 tildes, in the form of:
>
> ````
> ~~~~exercism/note
> Content goes here
>
> You can include code:
> ```ruby
> str = "Hello, World"
> ```
> ~~~~
> ````
>
> (Note: You may also use backticks or other levels of tildes in exceptional circumstances)