doc: explain how to use triggers in migrations (#4138)

examples/triggers/schema.sql

@@ -0,0 +1,29 @@
+-- Function to audit changes in the users table.
+CREATE OR REPLACE FUNCTION audit_users_changes()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF (TG_OP = 'INSERT') THEN
+        INSERT INTO user_audit_logs(operation_type, operation_time, new_value)
+        VALUES (TG_OP, CURRENT_TIMESTAMP, row_to_json(NEW));
+        RETURN NEW;
+    ELSIF (TG_OP = 'UPDATE') THEN
+        INSERT INTO user_audit_logs(operation_type, operation_time, old_value, new_value)
+        VALUES (TG_OP, CURRENT_TIMESTAMP, row_to_json(OLD), row_to_json(NEW));
+        RETURN NEW;
+    ELSIF (TG_OP = 'DELETE') THEN
+        INSERT INTO user_audit_logs(operation_type, operation_time, old_value)
+        VALUES (TG_OP, CURRENT_TIMESTAMP, row_to_json(OLD));
+        RETURN OLD;
+    END IF;
+    RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+-- Trigger for INSERT operations.
+CREATE TRIGGER users_insert_audit AFTER INSERT ON users FOR EACH ROW EXECUTE FUNCTION audit_users_changes();
+
+-- Trigger for UPDATE operations.
+CREATE TRIGGER users_update_audit AFTER UPDATE ON users FOR EACH ROW EXECUTE FUNCTION audit_users_changes();
+
+-- Trigger for DELETE operations.
+CREATE TRIGGER users_delete_audit AFTER DELETE ON users FOR EACH ROW EXECUTE FUNCTION audit_users_changes();