Migrate eddyqc section from FSL wiki

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

The suggested group for this section is Diffusion MRI:

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

• eddyqc/Contents
• eddyqc/UsersGuide
• eddyqc