Migrate FIRST section from FSL wiki

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

The suggested group for this section is Structural MRI:

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

• FIRST/UserGuide
• FIRST
• FIRST/StepByStep
• FIRST/Contents

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/FIRST(2f)UserGuide