Our Pet of the Week, Sienna, has officially retired from motherhood, and at just 2 years old, she’s more than ready for the next chapter: being somebody’s baby instead. At about 40 pounds, Sienna is a beautifully compact, medium-sized girl with a gorgeous brindle coat, a bold white chest, soulful dark eyes, and one of those wonderfully expressive faces that seems to change with every thought. She can look dignified and serious one second, then f…