Label steps in the Getting Started section

[johnpryan]

We should consider labeling each major section as a step in the Getting Started docs, for example:

• Step 1 System requirements
• Step 2 Configure editor or integrated development environment
• Step 3 Install Flutter SDK
• Step 4 Configure for [Android/iOS/Web/Desktop] development
• Step 5 Check your development setup

I don't see very many examples of this in Google Developer documentation, but SwiftUI tutorials do show section numbers and step numbers within a section.