🎨 [Guide Re-org] - Update `Getting Started`

[sbddesign]

Update pages in the “Getting started” section. New structure:

• Why bitcoin is unique
• Why design for bitcoin (moved from “Designing bitcoin products”)
• Open design (moved from “Designing bitcoin products”)
• Visual language
• Technology primer
• Design principles (moved from “Designing bitcoin products”)
• Software categories (renamed from “Software overview”)
• Hardware categories (renamed from “Hardware overview”)

This is an idea that came out of our brainstorming session on re-organizing the Guide content.