Fix quick start documentation (#249)
Fix quick start documentation (#249)

1. `gripper.hide_visualization()` is deprecated.
2. `get_collision` is deprecated and should be replaced by `get_obj`.
3. there's already a free joint that is enabled by default. Adding an
extra one lead to error when `model = world.get_model(mode="mujoco_py")`

Co-authored-by: Zhenyu Jiang <282d5aced935096acb8e19ae7241138ca6cd20b5@cs.utexas.edu>
ARISE-Initiative/robosuite