Migrate Contributors section from FSL wiki

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

The suggested group for this section is Development:

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

This section has the following subpages on the old wiki:

• Contributors