Documentation Update: Expand tutorial to explain run_metrics pipeline

[cmalinmayor]

We want to explain the behind the scenes of run_metrics, showing an example in the tutorial where we instantiate Matcher/Metrics objects and call the API explicitly. This is both educational and potentially useful to users who want to customize their pipelines or examine intermediate objects like matchings. These snippets could also or alternatively be incorporated in the "How It Works" section (currently titled "Getting Started").