Change "Introduction" to "Why Bean Machine?" (#1173)

Summary:
Pull Request resolved: facebookresearch/beanmachine#1173

Changed "Introduction" to "Why Bean Machine?" because "Introduction" is too similar to "Quick Start" and is really about motivation for Bean Machine.

Reviewed By: jpchen, michaeltingley

Differential Revision: D32900373

fbshipit-source-id: c3c682bdbb1ed4ffceaae689850b9f960bfca91b

docs/getting_started.md

@@ -4,7 +4,7 @@ title: Getting Started
 ---
 This document outlines how to get started with PPL Bench.
 
-Before jumping into the project, we recommend you read [Introduction](introduction.md) and [System Overview](system_overview.md) documents.
+Before jumping into the project, we recommend you read ["Why Bean Machine?"](why_bean_machine.md) and [System Overview](system_overview.md) documents.
 
 ## Installation