docs: fix MCP host configuration dev guide

Corrected 12 critical issues in the MCP host configuration extension documentation:

CRITICAL FIXES:
- Added missing is_host_available() method to core interface (was causing NotImplementedError)
- Added missing get_config_key() method to interface and examples
- Fixed incorrect claim that Claude requires absolute paths (actually accepts any command/URL)
- Clarified that base classes provide full read/write implementations

IMPORTANT ADDITIONS:
- Listed all existing host types (CLAUDE_DESKTOP, CLAUDE_CODE, VSCODE, CURSOR, LMSTUDIO, GEMINI)
- Added comprehensive error handling examples with try-catch patterns
- Added atomic write pattern to prevent config file corruption
- Added preservation of non-MCP settings in config files
- Clarified backup handling is managed by MCPHostConfigurationManager, not strategies
- Added explanation of get_config_key() and when to override it
- Added import requirement for CLI discovery

IMPROVED EXAMPLES:
- Split implementation examples into two paths: inheriting from base classes vs. from scratch
- Added platform-specific path detection with platform.system()
- Added nested configuration structure handling
- Added multi-file configuration support
- Added host-specific validation examples
- Improved test examples with proper imports and more test cases

DOCUMENTATION IMPROVEMENTS:
- Clarified that base classes inherit read/write logic
- Added explanation of registration mechanism
- Added CLI import requirement explanation
- Improved section on common patterns with better organization

Author: LittleCoinCoin