Migrate lesion_filling section from FSL wiki

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

The suggested group for this section is Structural MRI:

• The section content should be migrated into docs/structural/lesion_filling.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:

• lesion_filling