include pointer to fsl_deface

Just discovered that [FSL](https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FSL) now also has a defacing tool which is based on the one from [UK Biobank](https://www.sciencedirect.com/science/article/pii/S1053811917308613). It's already included being called fsl_deface, however apparently not working properly (e.g., [see here](https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=FSL;4fea3c4.1903)). Once it has been tested and working properly, a pointer should be included in the [anonymization tools section of the docs](https://open-brain-consent.readthedocs.io/en/stable/anon_tools.html).

Start by checking whether FSL's fsl_deface works properly, using the linked FSL and UK Biobank references and the reported failure example. Once verified, update the anonymization tools section of the documentation with a pointer to fsl_deface and confirm the documentation reflects the tested tool.

Written by the indexing model from the issue text.