Migrate FEAT section from FSL wiki

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

The suggested group for this section is Task fMRI:

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

This section has the following subpages on the old wiki:

• FEAT/UserGuide
• FEAT/FAQ
• FEAT
• FEAT/StepByStep1
• FEAT/GuideStats
• FEAT/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/FEAT(2f)FAQ
• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/FEAT(2f)UserGuide