doc: fix build when sphinx reports version to stderr

When sphinx-build reports its version information to stderr rather
than stdout, the wrapper script misses it, and then fails to run.
We can fix this by redirecting stderr to stdout for the version
query call.

Fixes: f5ab2074cfba ("doc: rebuild with meson whenever a file changes")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Ciara Power <ciara.power@intel.com>
Tested-by: David Marchand <david.marchand@redhat.com>