Bump version to 1.5.0 with comprehensive documentation and plugin system

Major features and improvements:
- Fix critical guide template variable replacement bug
- Complete guide documentation rewrite with examples
- Implement plugin system with HTMLBlock and MQTTTracker plugins
- Enhanced bilingual support (NL/EN) throughout the system
- Improved template system with better layout options
- Enhanced security headers and code quality
- Updated documentation and configuration examples

Version 1.5.0 represents a significant milestone with improved
documentation, plugin architecture, and bug fixes.

README.en.md

@@ -4,7 +4,7 @@
 
 A lightweight, file-based content management system built with PHP.
 
-**Version:** 1.0.0 | **License:** AGPL v3 / Commercial
+**Version:** 1.5.0 | **License:** AGPL v3 / Commercial
 
 ## ✨ Features