Minor Labrinth documentation and missing Sendy configuration handling tweaks (#3667)

* docs(contributing/labrinth): mention SMTP vars, add helpful Redis note

* chore(labrinth): short-circuit Sendy usage successfully when not setup

* chore: get rid of deprecated `docker-compose` version parameter

docker-compose.yml

@@ -1,4 +1,3 @@
-version: '3'
 services:
   postgres_db:
     image: postgres:alpine