Fix broken sphinx-codeautolink API links with `dirhtml` builder

Since https://github.com/ComPWA/policy/issues/158, [`sphinx-codeautolink`](https://sphinx-codeautolink.readthedocs.io) does not work anymore. This is probably because the [`dirhtml`](https://www.sphinx-doc.org/en/master/usage/builders/index.html#sphinx.builders.dirhtml.DirectoryHTMLBuilder) builder is not supported.