Improve React on Rails documentation for v16 and coding agents (#1789)

* Improve React on Rails documentation for v16 and coding agents

## New Documentation
- **troubleshooting-build-errors.md**: Comprehensive guide for webpack/build issues
  - Missing routes file error patterns and solutions
  - ProvidePlugin module resolution troubleshooting
  - Environment setup dependencies and workarounds
  - Agent-friendly diagnostic scripts and auto-fixes

- **coding-agents-guide.md**: Structured workflows for AI coding agents
  - Version compatibility matrix
  - Installation and upgrade workflows with error detection
  - Auto-fix strategies for common issues
  - Emergency procedures and rollback guidance

## Updated Documentation
- **upgrading-react-on-rails.md**:
  - Removed js:export from upgrade steps (it's a setup issue, not upgrade-specific)
  - Focused troubleshooting on actual upgrade-related issues
  - Added reference to comprehensive build errors guide

- **getting-started.md**: Updated to use react_on_rails v16.0.0
- **home.md**: Added troubleshooting guide to navigation

## Key Improvements
- Separated setup/installation issues from upgrade-specific issues
- Provided agent-friendly automation and error detection
- Enhanced troubleshooting with actionable solutions
- Improved documentation discoverability

These improvements make react_on_rails documentation more comprehensive
and accessible for both human developers and AI coding agents.

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

Co-Authored-By: Claude <noreply@anthropic.com>

* Clarify js-routes as optional dependency in documentation

## Problem
Documentation gave the impression that js-routes was a core react_on_rails
requirement, when it's actually an optional integration used only in specific
scenarios.

## Changes Made

### troubleshooting-build-errors.md
- Added clear note that missing routes error only occurs with js-routes gem
- Explained when js-routes IS and ISN'T needed
- Provided alternative solution: remove ProvidePlugin if not using js-routes
- Distinguished modern SPA patterns from Rails-heavy integration patterns

### coding-agents-guide.md
- Added "(if using js-routes gem)" qualifier to all js:export commands
- Updated auto-fix functions to clarify js-routes context
- Modified diagnostic scripts to indicate js-routes dependency

### Environment dependencies
- Clarified that rails js:export is only needed for js-routes gem

## Result
- Documentation now accurately reflects js-routes as optional
- Developers won't be confused into thinking it's required for react_on_rails
- Clear guidance on when to use js-routes vs modern alternatives
- Better separation of concerns between react_on_rails core and optional integrations

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

Author: justin808