Migrate FDR section from FSL wiki

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

The suggested group for this section is Statistics:

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

This section has the following subpages on the old wiki:

• FDR