Add cpac comparison run docs

tests/data/.gitignore

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+cpac_outputs

tests/data/CPAC.md

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+# C-PAC Test Comparison Run
+
+Run C-PAC on the test dataset with minimal parallelization:
+
+```sh
+docker run --rm -it \
+  -v $(pwd)/tests/data/ds000001:/bids_input:ro \
+  -v $(pwd)/tests/data/cpac_outputs/ds000001:/outputs \
+  fcpindi/c-pac:release-v1.8.5.dev1 \
+  /bids_input /outputs participant \
+  --n_cpus 1 \
+  --num_ants_threads 1 \
+  --skip_bids_validator \
+  --preconfig rbc-options \
+  --save_working_dir
+```
+
+This mounts the BIDS dataset as read-only input and saves outputs and intermediates locally.
+
+Clean up outputs:
+
+```sh
+rm -rf tests/data/cpac_outputs/ds000001/
+```