Mesa generates a number of headers during build. To make Clang based
documentation parsing work without depending on the generated headers,
start a directory hierarchy under docs/header-stubs/ for static mock
headers.
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24507>
