Docs: Update Quickstart / getting started demo language

[alejandro-ponder]

https://github.com/modin-project/modin/blame/master/docs/getting_started/quickstart.rst#L51

"For the purpose of demonstration, we will load in modin as pd and pandas as pandas."

Before jumping into the mechanics of loading Modin, consider introducing the demo problem here. It was very well written in the summary at the end of the section, maybe worth including it up front here before talking about import statements. "this tutorial demonstrated how Modin delivers significant speedup on pandas operations without the need for any extra effort. Throughout example, we moved from working with 100MBs of data to 20GBs of data all without having to change anything or manually optimize our code to achieve the level of scalable performance that Modin provides."