docs: Filter missing static directories in conf.py
docs: Filter missing static directories in conf.py

sphinx-build warns when '_static' doesn't exist so filter it out.
This'll also allow us to have a _static directory in the future
without having to change conf.py.

Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>

dongguangming/jsonpickle,mandx/jsonpickle,mandx/jsonpickle,dongguangming/jsonpickle,dongguangming/jsonpickle,mandx/jsonpickle,dongguangming/jsonpickle,mandx/jsonpickle