Rename 'stored procedures' to 'user-defined functions'
Closes #147
On stand-by until https://github.com/PostgREST/postgrest/issues/1210 is resolved.
The term user-defined function does come from PostgreSQL docs but I don't think it's commonly used(I've seen UDF at some point though) - this might hurt searchability/SEO. Perhaps we can just call them "Functions"?
Though we can link to the above in the "Functions" section to be aligned with pg docs.
On stand-by until https://github.com/PostgREST/postgrest/issues/1210 is resolved.
Since this issue is on-hold for now and the PR here is heavily outdated.. I will just close it. It would need a major rebase anyway, because the whole structure has changed a lot in the last three years. It's probably simpler to just redo it by now.