tutorials: improve joint mobilization visuals #23
Description
Make joint mobilization diagrams clearer for concrete joints
Ensure the prismatic initial conditions are specified for joint kinematics; make Drake agree
Came about when debugging: RobotLocomotion/drake#12775
In kinematics tutorial: http://sdformat.org/tutorials?tut=spec_model_kinematics&cat=specification&
Would be nice to have concrete rendering of each joint, with pose offsets and small mobilization (with zero offset)