Simplify configuration files to improve usability

After PR #1993 introduced sensible defaults for advanced options like
generated_component_packs_loading_strategy (auto-configured based on Pro
license), we can significantly simplify generated configuration files
and documentation to make React on Rails easier to use.

Changes:

**Generator Template Simplification:**
- Reduced react_on_rails.rb.tt from 67 to 42 lines
- Removed redundant explanations for options with good defaults
- Added clear pointer to docs for advanced configuration
- Kept only essential options in generated files
- Commented out file-system component registry (optional feature)

**Documentation Restructuring:**
- Added "Quick Start" section showing minimal config
- Reorganized into "Essential" vs "Advanced" configuration
- Documented which options auto-configure based on Pro license
- Added clear defaults and recommendations for each option
- Improved searchability with structured sections
- Added deprecation section for immediate_hydration

**Essential Configuration (in generated files):**
- server_bundle_js_file - Required for SSR
- build_test_command - Used for testing
- components_subdirectory - Optional file-based registry
- auto_load_bundle - Optional auto-loading

**Advanced Configuration (docs only, sensible defaults):**
- generated_component_packs_loading_strategy (auto: :async for Pro, :defer for non-Pro)
- server_bundle_output_path (default: "ssr-generated")
- enforce_private_server_bundles (default: false)
- All rendering pool, debugging, and customization options

**Dummy App Updates:**
- Updated all dummy app configs with clear comments
- Separated essential from test-specific configuration
- Maintained same functionality with better organization

Benefits:
- New users see only 2-3 required config options
- Clear path from simple to advanced configuration
- Reduced cognitive load and decision fatigue
- Better reflects PR #1993 auto-configuration improvements
- Easier to understand what needs customization

Closes: Follow-up to PR #1993

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808