Added Light-sheet challenge data

[ricardo-re-azevedo]

Updated main page to include the light sheet data as a second challenge dataset. I did some restructuring to the challenge section to make the challenge datasets a subsection. May be good for @lydiang to review as well. I restructured the fMOST section but dont want to overstep.

[dzenanz]

This looks like it needs rebasing. The contents seems to be triplicated with minor variation.

[ricardo-re-azevedo]

This looks like it needs rebasing. The contents seems to be triplicated with minor variation.

Fixed