doc: allow external build

doc: allow external build

Currently, when building sphinx documentation, the build will only
succeed if being run from the build system, because the conf.py script
expects DPDK_VERSION environment variable to be set, and crashes if it
is not.

However, there are certain external tools (such as sphinx documentation
preview extensions for certain IDE's) that use live preview and thus
rely on running their own sphinx commands. In these cases, it is useful
to permit building sphinx documentation without specifying the
DPDK_VERSION environment variable. The version string is the only thing
preventing manual sphinx build commands from working.

Fix the conf.py to use "None" as a version string in cases when
DPDK_VERSION environment variable is not set.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>