Add descriptions.tsv documenting desc-<label> values

[CMonnin]

Follow-up from the BIDS-derivatives review on #11 (deferred to a separate PR).

BIDS recommends a descriptions.tsv documenting each desc-<label> value the pipeline introduces.

desc labels currently emitted

• denoised
• biasCorrection
• maskClassified
• registration
• affine
• nonlinear
• rigid

Notes

• Columns: desc_id, description -- confirm exact column names against the current spec before implementing.
• Dataset-level file (one per dataset, not per output).

Spec

• https://bids-specification.readthedocs.io/en/stable/derivatives/imaging.html