Migrate DualRegression section from FSL wiki

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

The suggested group for this section is Resting state fMRI:

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

This section has the following subpages on the old wiki:

• DualRegression/StepByStep
• DualRegression/Contents
• DualRegression/Faq
• DualRegression/UserGuide
• DualRegression

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/DualRegression