models: Should migrate data to RobotLocomotion/models per doc advice

[EricCousineau-TRI]

Follow-up to #10811.

To relocate:

• examples/atlas/urdf/meshes
• examples/pr2/models/pr2_description/meshes
• manipulation/models/allegro_hand_description/meshes
• manipulation/models/iiwa_description/iiwa7
• manipulation/models/iiwa_description/meshes

To deprecate and remove:

• examples/irb140/urdf/meshes (examples: Deprecate IRB140 and big_robot_toy models #13831)
• examples/kuka_iiwa_arm/models/objects/meshes (examples: Deprecate IRB140 and big_robot_toy models #13831)
• manipulation/models/wsg_50_description/meshes (wsg_50_description: Deprecate the URDF and meshes #13693)

[EricCousineau-TRI]

FYI @IanTheEngineer

[jwnimmer-tri]

This is confusing a few new people every month, so I'm going to work on it.

[jwnimmer-tri]

\CC @IanTheEngineer

[jwnimmer-tri]

It's possible that the resolution of #13942 will end up being a prerequisite for this work. If we decide that the sdformat / urdf files should move as well, it probably makes sense to do them as part of the same file relocation as the meshes.

The package URIs per #10531 might also come into play.

[jwnimmer-tri]

The only remaining one to port is the PR2. It's complicated by the fact that we're currently reprocessing those models with stl2obj and then installing the reprocessed copies. The easy fix is to commit the reprocessed files into the models repository. It might be worth seeing if there is any a more elegant solution available instead.

[jwnimmer-tri]

I'll roll the one remaining task into #13942, so we can close this one.