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: https://github.com/RobotLocomotion/drake/issues/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)