Migrate fsl_anat section from FSL wiki

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

The suggested group for this section is Structural MRI:

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

This section has the following subpages on the old wiki:

• fsl_anat