I had never heard of Kate Nash, the singer, songwriter, actor, and subject of “Underestimate the Girl,” a documentary by filmmaker Amy Goldstein. I’m now a fan. Nash is an irresistibly likeable subject, and she is a huge part of what makes the film so entertaining. This is a great doc that explores what it’s like to be a musician in the modern world. This polished film tells the story of Nash, tackling the entire trajectory of her career. Starting from her first taste of superstardom at age 18 to a decade later when she is nearly homeless, the film offers an intimate look at the challenges,...