Clean up PR2 model (#20588)

- Minor collision geometries (often described by seemingly arbitrary
  small boxes) have been commented out; they were causing
  self-collisions even in the default configuration.

- Additional Drake collision-filtering tags have been added to remove
  remaining self-collisions in the default position.

- The finger tip collision mesh geometry has been replaced by simple
  box geometry, with additional Drake tags to support hydroelastic
  contact.

- Long comments we reflowed to fit in Drake's preferred 80 character
  line limit.

examples/pr2/README.md

@@ -1,16 +1,3 @@
-This directory contains a drake-compatible description of the PR2.
-
-The description differs from a standard PR2 description because it uses two
-prismatic joints and one revolute joint to move along the ground plane (named x,
-y, and theta, respectively), instead of wheels. The description also contains
-fully actuated finger joints, instead of a single prismatic joint that closes
-and opens finger joints that mimic each other.
-
-In addition, some tags unsupported by Drake have been removed, to reduce the
-burden of warning output. For URDF support details, see:
-https://drake.mit.edu/doxygen_cxx/group__multibody__parsing.html
-
-[1] To view or use the C++ passive simulation of a PR2 based on RigidBodyTree
-(removed as of 2020), you may use this commit:
-
-https://github.com/RobotLocomotion/drake/tree/v0.14.0/examples/pr2
+This directory contains a drake-compatible description of the PR2. See the
+comment at the top of the urdf for an inventory of changes from the original
+model.