Deprecate datafile/pre/@source #2755
Description
The datafile/pre/@source construct ends up being awkward to use if the contents of the source file need XML escaping. The attribute also adds confusion as it is only available in the context of a datafile and means something different (build time file inclusion) than most uses of @source (run time file inclusion).
Examples should point to using xi:include
See #2615 for discussion.