The declaration is the element with `<?xml version="..."?>`.
https://www.w3.org/TR/xml11/#sec-prolog-dtd
It's required for XML 1.1. However, we'll always be using XML 1.0 because that's all libxml2 supports.
https://discourse.gnome.org/t/does-xmlnewdoc-support-xml-1-1/19329
See also https://github.com/ClusterLabs/pacemaker/pull/3323#discussion_r1457733427
---
If we do this, perhaps advertise UTF-8 encoding as well, something like
```
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
```