Update documentation (and code if needed) to make placement constraints 3D

[vaughnbetz]

VTR now supports 3D devices, but the placement constraints documentation at https://docs.verilogtorouting.org/en/latest/vpr/placement_constraints/ is still 2D.

Proposed Behaviour

Update the example and documentation to 3D. Confirm the code works for 3D devices. Add a regtest for a few 3D placement constraints (e.g. one to a specific x,y,layer, and another to a region of (xlow-->xhigh, ylow-->yhigh, layer_low-->layer_high).

[vaughnbetz]

Also we need a test (can be a small device) with a few 3D constraints.

• One specific one: x,y,layer,subtile
• At least one larger one: (xlow->xhigh, ylow->yhigh, layer_low->layer_high)
• Probably one with the union of two regions.