refactor(.env.example): restructure and categorize environment variables for better readability and understanding

feat(.env.example): add comments to clarify usage of each environment variable for better user guidance
style(.env.example): remove unused SUPABASE_KEY variable to clean up the code

.env.example

@@ -1,20 +1,27 @@
+#
+# Required
+#
+
+# If in production mode:
+
+# DEV=False
 PROD_DATABASE_URL=""
-# PROD_SUPABASE_DB_PASSWORD=""
 PROD_TOKEN=""
 
-DEV=True
+# If in development mode:
+
+# DEV=True
 DEV_DATABASE_URL=""
-# DEV_SUPABASE_DB_PASSWORD=""
 DEV_TOKEN=""
 
-TZ="UTC"
-DISCORD_GUILD=
+#
+# Optional
+#
 
-SUPABASE_KEY=""
 SENTRY_URL=""
 
-# PROD_COG_IGNORE_LIST="kick,ban"
-# DEV_COG_IGNORE_LIST="mail, git"
+PROD_COG_IGNORE_LIST=
+DEV_COG_IGNORE_LIST=
 
 GITHUB_APP_ID=
 GITHUB_CLIENT_ID=""