ResourceStack: nest stacked plates by Plate.stacking_z_height

[c-reiter]

Summary

Follow-up to #1110, which added Plate.stacking_z_height. This PR makes ResourceStack actually use it, and brings in the documentation that belonged with that work.

Why a separate PR: these changes were force-pushed to the #1110 branch a few minutes after it had already been squash-merged, so they never reached main. #1110 landed only the Plate attribute (param, serialize(), __eq__/__hash__, tests); the ResourceStack nesting and the docs were left behind on the merged branch. This PR re-applies that delta on top of current main.

Changes

• ResourceStack: bare plates stacked in the z direction now nest by their stacking pitch. A plate placed directly on another bare plate sinks in by size_z - stacking_z_height, so a stack of N identical plates is size_z + (N - 1) * stacking_z_height tall — applied consistently to both get_size_z() and child placement via a shared _nesting_overlap() helper. Gated on the value being set, and skipped when the lower plate wears a lid. This parallels the lid-nesting ResourceStack already performed.
• Docs: documented stacking_z_height (what it is, how to measure it, the ResourceStack nesting behavior) on the Plate resource page and the ResourceStack notebook, and added it to the example plate definition in the contributor guide.
• Tests: two/three plates nest to the correct height and positions, no-nesting without the value (back-compat), and no nesting onto a lidded plate.

Notes

• Backwards compatible: no library plate sets stacking_z_height, so ResourceStack behavior is unchanged for all existing labware.

[rickwierenga]

thanks!