contributing: Fix lua stript

The lua script for filtering out toctrees started to fail with the latest pandoc with:

./CONTRIBUTING.lua:12: attempt to index a nil value (field 'integer index')

We fix it by using the classes:includes which is simpler and it works.

requested review from @stefanschmidt, @mrybczyn, and @alpianon

mentioned in merge request oniro!359 (merged)

approved this merge request

approved this merge request

merged