docs: add presets reference page and rename pack_id to preset_id (#2243)

- New docs/presets.md: command reference for all 9 specify preset commands
  and 3 specify preset catalog commands, file resolution stack with Mermaid
  diagrams, catalog resolution order, and FAQ
- src/specify_cli/__init__.py: rename pack_id to preset_id across all preset
  CLI commands so --help shows PRESET_ID matching the docs
- docs/toc.yml: add Presets under Reference section
- README.md: update presets link to published docs site