these files provided mainly or exclusively xml-related tooling and
support for editing the manual. since docbook is now an implementation
detail (except for still being allowed in option docs, for now) these
tools are no longer necessary, useful, or even functional.
xmlformat.conf is still used by the nixpkgs documentation, so we have to
keep it. there's no reason it can't go live with the nixpkgs docs though.
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