Raw BIDS support in the laion_fmri dataloader

[PeerHerholz]

• Adds raw-BIDS support to the laion_fmri dataloader (multi-echo BOLD, sbref, fieldmaps, raw MEGRE, and per-run events.tsv)
• Two entry points: download(..., include_raw=True) for additive fetches, and download_raw(...) (plus laion-fmri download-raw CLI) for raw-only
• Subject.has_raw, Subject.get_events, Subject.get_raw_bold, Subject.get_sbref cover the load side.
• Discovery now surfaces subjects present only under sub-XX/ (raw-only uploads)
• Backfills the previously Python-only --include-freesurfer and --include-anatomical CLI flags
• Docs: new "Raw BIDS data" section in data_access.rst, raw loading section in fmri_data.rst, download_raw API reference, and a raw-events cell in plot_04_loading (plus callouts in plot_01 and plot_03)
• Gallery build: conf.py preflights download_stimuli() so the rendered gallery has real stimulus-derived output