Support for footnote syntax

I'd love to see support for footnotes, e.g.

```markdown
The system can only support single-environment deployment.
This limits our capacity.
While we work on a solution[^prod_pr_open],
we'll continue to adjust resources in the primary environment.

[^prod_pr_open]: Work for deployment to multiple environments exists in a draft PR.
```

renders as such in GitHub, GitLab, goldmark, pandoc, and others (but is not yet a part of [Commonmark](https://spec.commonmark.org)):

---

The system can only support single-environment deployment.
This limits our capacity.
While we work on a solution[^prod_pr_open],
we'll continue to adjust resources in the primary environment.

---

[^prod_pr_open]: Work for deployment to multiple environments exists in a draft PR.