doc: Add empty .gitignore for source/_static
doc: Add empty .gitignore for source/_static

Git doesn't track directories so this is the usual procedure.

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

dongguangming/jsonpickle,eoghanmurray/jsonpickle_prev,eoghanmurray/jsonpickle_prev,mandx/jsonpickle,dongguangming/jsonpickle,dongguangming/jsonpickle,mandx/jsonpickle,mandx/jsonpickle,mandx/jsonpickle,dongguangming/jsonpickle