Switch the markdown parser to the one recommended by the current Sphinx
documentation [1].

This fixes an error parsing Markdown code blocks in `adr-003.md` when
running `make checkdocs` and makes this file show up under "Architecture
Decision Records" in the generated docs.

The `requirements/docs.txt` file was updated by directly running
`.tox/docs/bin/pip-compile -r requirements/docs.in`.

[1] https://www.sphinx-doc.org/en/master/usage/markdown.html