docs: update output directory structure to reflect naming refactor

[Copilot]

The large naming refactor split outputs across multiple typed subdirectories instead of placing everything under micr/. Documentation still showed the old flat micr/ structure for all outputs.

Updated docs

• docs/reference/outputs.md — Replaced single-folder diagram with accurate multi-folder tree; added per-folder sections describing each output type; fixed .nii → .nii.gz for group volume maps
• docs/faq.md — Updated "Where are my results?" to show the new structure with inline folder descriptions
• docs/getting_started/quickstart.md — Updated Step 5 output tree with realistic filenames and corrected key-files list

New output structure

output/spimquant/
├── sub-01/
│   ├── micr/       # registered NIfTIs + masks + regqc HTML
│   ├── seg/        # full-res segmentation OME-Zarr
│   ├── parc/       # atlas dseg NIfTIs  (was micr/)
│   ├── tabular/    # segstats, regionpropstats, fieldfracstats, countstats  (was micr/)
│   └── xfm/        # affine + deformable transforms
└── group/          # groupstats TSV/PNG/NIfTI