The Doxygen configuration, the page list, and the docs target live in
docs/, and the written documents live in docs/PROSE/, leaving the
top-level file with the output paths and the add_subdirectory call.
The subdirectory is given its own binary directory so that CMake's
scaffolding stays out of the documentation output, which holds pdf/ and
html/ and nothing else.
Paths both files need are set once above the call: the child's output
and cleanup and the parent's clean rule refer to the same variables.
