Migrate POSSUM section from FSL wiki

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

The suggested group for this section is Utilities:

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

This section has the following subpages on the old wiki:

• POSSUM/Contents
• POSSUM/UserGuide
• POSSUM

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/POSSUM(2f)UserGuide