Misc: examples folder rationalization, benchmarking, more trajectory generators, gripper composition #677

Grab-bag of small housekeeping/feature ideas, migrated from the wiki's Contributing and Grippers pages.

• Rationalize the examples/ folder (grown organically, could use a pass for consistency/dedup)
• Benchmarking and performance optimisation pass
• More trajectory generators, e.g. a proper multi-segment point-to-point generator (mstraj covers waypoints, but there may be room for more)
• Attaching an existing Gripper (or other end-effector subtree) from one robot model onto a different robot model isn't a documented/ergonomic workflow today — Gripper and robot.grippers are real and shipped, but built at model-construction time; there's no supported way to e.g. bolt a UR gripper model onto a Panda model after the fact