Add 'no ai slop' to community guidelines

Apparently, machine-generated content already violates our ToS: Proposal: Unity Asset Store or GitHub Package Integration - #14 by jamshark70

By making Content available, you represent and warrant that: … the Content is not spam, is not machine- or randomly-generated…

Does this apply to AI generated posts? (IMO it does.) If so we should make it explicit in the ToS.

(Side note: for some reason I can’t access https://scsynth.org/t/terms-of-service. Anyone knows why?)