eyehandcal - where to place markers?

## Type of Issue

Select the type of issue:
- [ ] Bug report (to report a bug)
- [ ] Feature request (to request an additional feature)
- [ ] Tracker (I am just using this as a tracker)
- [ ] Refactor request
- [X] Documentation Ask

## Description

In the current documentation for `eyehandcal` it’s not immediately apparent where to place the Aruco marker for single/multi-cam eye to hand calibration (cameras external to robot).

Do we mount it on the Franka hand/Robotiq hand? What are the desiderata (should be visible to at least one camera at all times/all poses?)

Some pictures/video demos of what the calibration process looks like would be amazing! Happy to contribute that if you could help me with these questions!