Add docs to gitignore.
Add docs to gitignore.

Documentation tools create this folder that shall not be uploaded to git.

wenjiebit/velodyne,vrabaud/velodyne,wenjiebit/velodyne,vrabaud/velodyne,wenjiebit/velodyne,lemiant/velodyne,lemiant/velodyne,lemiant/velodyne,vrabaud/velodyne