feat: add GitHub Alerts transformer and renderers

Co-Authored-By: Manuel Rüger <manuel@rueg.eu>

transformer/README.md

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+# GitHub Alerts Transformer
+
+This directory contains the GitHub Alerts transformer that enables Mark to convert GitHub-style alert syntax into Confluence macros.
+
+## Overview
+
+The GitHub Alerts transformer processes markdown with GitHub Alert syntax like `[!NOTE]`, `[!TIP]`, `[!WARNING]`, `[!CAUTION]`, and `[!IMPORTANT]` and converts them into appropriate Confluence structured macros.
+
+## Supported Alert Types
+
+| GitHub Alert | Confluence Macro | Description |
+|--------------|-----------------|-------------|
+| `[!NOTE]` | `info` | General information |
+| `[!TIP]` | `tip` | Helpful suggestions |
+| `[!IMPORTANT]` | `info` | Critical information |
+| `[!WARNING]` | `note` | Important warnings |
+| `[!CAUTION]` | `warning` | Dangerous situations |
+
+## Usage Example
+
+### Input Markdown
+
+```markdown
+# Test GitHub Alerts
+
+## Note Alert
+
+> [!NOTE]
+> This is a note alert with **markdown** formatting.
+> 
+> - Item 1
+> - Item 2
+
+## Tip Alert
+
+> [!TIP]
+> This is a tip alert.
+
+## Warning Alert  
+
+> [!WARNING]
+> This is a warning alert.
+
+## Regular Blockquote
+
+> This is a regular blockquote without GitHub Alert syntax.
+```
+
+### Output (Confluence Storage Format)
+
+The transformer converts GitHub Alert syntax into Confluence structured macros:
+
+```xml
+<ac:structured-macro ac:name="info">
+  <ac:parameter ac:name="icon">true</ac:parameter>
+  <ac:rich-text-body>
+    <p>Note</p>
+    <p>This is a note alert with <strong>markdown</strong> formatting.</p>
+    <ul>
+      <li>Item 1</li>
+      <li>Item 2</li>
+    </ul>
+  </ac:rich-text-body>
+</ac:structured-macro>
+```
+
+## Key Features
+
+- **GitHub Compatibility**: Full support for GitHub's alert syntax
+- **Markdown Preservation**: All markdown formatting within alerts is preserved
+- **Fallback Support**: Regular blockquotes without alert syntax remain unchanged
+- **User-Friendly Labels**: Adds readable labels (Note, Tip, Warning, etc.) to alert content
+- **Confluence Integration**: Maps to appropriate Confluence macro types for optimal display
+
+## Implementation
+
+The transformer works by:
+
+1. **AST Transformation**: Modifies the goldmark AST before rendering
+2. **Pattern Matching**: Identifies GitHub Alert patterns in blockquotes
+3. **Content Enhancement**: Adds user-friendly labels and processes nested markdown
+4. **Macro Generation**: Converts to appropriate Confluence structured macros
+
+## Backward Compatibility
+
+- Legacy `info:`, `tip:`, `warning:` syntax continues to work
+- Regular blockquotes remain unchanged
+- Full compatibility with existing Mark features
+
+## Testing
+
+The transformer is thoroughly tested with:
+- All GitHub Alert types (`[!NOTE]`, `[!TIP]`, `[!WARNING]`, `[!CAUTION]`, `[!IMPORTANT]`)
+- Nested markdown formatting (bold, italic, lists, etc.)
+- Mixed content scenarios
+- Backward compatibility with legacy syntax
+- Edge cases and error conditions
+
+See `../markdown/transformer_comparison_test.go` for comprehensive test coverage.