Migrate FDT section from FSL wiki

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

The suggested group for this section is Diffusion MRI:

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

This section has the following subpages on the old wiki:

• FDT/Contents
• FDT/Pipeline
• FDT/FAQ
• FDT/UserGuide
• FDT/StepByStep
• FDT

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/FDT(2f)UserGuide