docs: Restructure DataMapper documentation to match Installation/Quickstart style

- Reorganized monolithic documentation into 5 numbered sections
- Converted all shortcodes from figure to image-sh for consistency
- Improved writing style to match Designer documentation
- Added Welcome and Getting Started sections
- Unified XML and JSON schema attachment process
- Enhanced XPath Editor section with comprehensive function reference
- Integrated resource management (delete/detach) into relevant sections
- Removed section 06 (integrated into other sections)
- Added inline icons for UI elements (trash, detach, layer icons)
- Cleaned up 20+ unused images and old files
- All internal links verified and updated

Sections:
01. Getting Started - Interface overview, parameters
02. Attaching Schemas - XML/JSON schema attachment
03. Creating Mappings - Drag-and-drop, XPath expressions
04. Conditional Mappings - if, choose-when-otherwise, for-each
05. XPath Editor - Expression editor with function reference"

Author: mmelko