Arm URDF is out-of-date (has 5 dofs)

[samzapo]

The URDF for the arm is out of date.

While the images used in the repo README are of the newer 6-dof arm, e.g.:

the URDF at simulation/low_cost_robot/low-cost-arm.urdf is of the older 5-dof arm, e.g.:

I've made an SDF of the new arm that may resolve this issue:
https://github.com/samzapo/gym-drake-lca/blob/main/gym_drake_lca/assets/follower_arm.sdf

[reedscot]

I have a PR that adds simulation for the 6-DoF arm: #41.