Supabase and anonymous user guide

[daniloc]

Via @ivanagas, citing a nice survey response:

Also we use Supabase, and they've introduced anon users, which would be a GREAT coupling with PostHog. You should look into that and write an article. 🤓

We don't currently have docs that exist near this particular intersection, so this would probably be a framework-specific guide, targeting Supabase, or maybe something more general about anonymous user handling with a Supabase example?