`nix eval --write-to` refuses to write to a directory that exists at
all, so now we generate in a temporary directory, and copy the generated
tree to the build directory. This is equivalent to what the Make
buildsystem did, actually, but hopefully more robust.
Future work: documenting the doc generation architecture in the
top-level meson.build outline comment.
Change-Id: Ic3eb6d26e3cc249a1c042fd3ced22d637ac66a69
