1. doc/source/_static will be auto generated when build docs and usually
cause difference with the original one after doc building.
2. manila/doc/.gitignore file is unnecessary because manila/.gitignore
already ignore the doc/build directory.
3. add git ignore of doc/source/_static in manila/.gitignore
Change-Id: Ibf13e08f0b0b57718d453ee3240d30124940b521