make index page for the reorganized mainspace/

[njr2128]

Create an index page for the newly restructured mainspace/ that lists each semeseter+year

This should then serve as the landing page for the new, browsable, reorganized version of the fieldnotes site, still at https://fieldnotes.makingandknowing.org/

[njr2128]

In CloudFront, point Default Root Object to this new index page

[gschare]

Index page could be a listing of each semester with dropdowns to view all subfolders and files.
Can be generated programmatically using the tree structure we have established from cu-mkp/fieldnotes-restructuring

[gschare]

See cf1c131

[gschare]

I played around with getting a nice collapsible menu but my lack of frontend experience quickly became an obstacle.