BOSTON — The Dallas Mavericks’ underdog bid to win the NBA Finals as a No. 5 seed ended in Game 5 on Monday night, as the Boston Celtics celebrated their 18th championship with a 106-88 win in TD Garden. Here are five thoughts from the loss: A champion is crowned In the game’s waning moments, NBA officials began to rope off the entrances to the parquet floor of TD Garden, the universal sign that…