Migrate FNIRT section from FSL wiki

Migrate the FNIRT section from the FSL wiki: https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FNIRT

The suggested group for this section is Registration:

• The section content should be migrated into docs/registration/fnirt.md.
• An entry for the new page should be added to docs/registration/_sidebar.md.

This section has the following subpages on the old wiki:

• FNIRT/UserGuide
• FNIRT/Contents
• FNIRT

Attachments for this section can be found at:

• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/FNIRT(2f)UserGuide
• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/FNIRT