Legacy Jekyll template `include gh_variables.html` remains in the front page

[cpauvert]

Hello!
First thanks for creating and maintaining this lesson! I spotted some legacy code on the front page of the lesson (cf. screenshot below). It was probably missed from the migration to {sandpaper}, as it is trying to include a HTML template that is no longer part of the repository {% include gh_variables.html %}.

Almost no other lessons seem to use the gh_variables template (but see geospatial-workshop with the old website template). Therefore, it seems it can be safely removed.

I will PR this soon.
Best,