Migrate SIENA section from FSL wiki

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

The suggested group for this section is Structural MRI:

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

• SIENA/FAQ
• SIENA/Contents
• SIENA
• SIENA/UserGuide