Migrate eddy section from FSL wiki

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

The suggested group for this section is Diffusion MRI:

• The section content should be migrated into docs/diffusion/eddy.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:

• eddy/UsersGuide
• eddy
• eddy/Contents
• eddy/Faq

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/eddy(2f)Faq
• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/eddy(2f)UsersGuide
• https://git.fmrib.ox.ac.uk/paulmc/fsl-docathon-2023/-/blob/main/fsl-wiki-dump-20230817/attachments/eddy