docs(scalebars): add rotation + label section, sync thumbnail

- New §5 demonstrating a vertical bar (`rotation`) and a custom bar title
  (`label`/`label_loc`); old wrap-up renumbered to §6.
- Regenerate the gallery thumbnail from this final plot: vertical 100 µm
  bar with a "scale" label, landscape ~1.11 to match the sibling cards.
- Re-executed end-to-end so committed outputs are current.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: timtreis