Transform getting-started.md into understanding-react-on-rails.md

Transform docs/getting-started.md (253 lines) into a focused conceptual
guide: docs/getting-started/understanding-react-on-rails.md (238 lines).

Changes:
- Removed redundant sections (Choose Starting Point, System Requirements, More Reading)
- Transformed installation section into brief conceptual overview with links
- Kept and refined: Basic Usage, Auto-Bundling, Render-Functions, Error Handling
- Added clear 'Next Steps' section with organized learning paths
- Updated link in quick-start.md to point to new file
- Deleted original docs/getting-started.md

Purpose: This is now a conceptual journey guide explaining how React on Rails
works, complementing the hands-on tutorial and quick-start guides. Addresses
user feedback that tutorial was too overwhelming and mixed audiences.

Next: Website config updates (keeping for last as requested)

Author: ihabadham

docs/getting-started.md

@@ -159,7 +159,7 @@ Now that you have React on Rails working, here's what to explore next:
 
 ### Immediate Next Steps
 
-1. **[Basic Configuration](../getting-started.md)** - Understand the setup
+1. **[Understanding React on Rails](./understanding-react-on-rails.md)** - Core concepts explained
 2. **[View Helpers API](../api-reference/view-helpers-api.md)** - Learn all the options for `react_component`
 3. **[Hot Module Replacement](../building-features/hmr-and-hot-reloading-with-the-webpack-dev-server.md)** - Optimize your dev workflow