Implement file preservation system for output directory cleaning

- Add sophisticated file preservation functionality to _clean_output_dir()
- Support .qsgen3_preserve file with shell glob patterns for selective preservation
- Preserve shared articles and important files during site regeneration
- Maintain backward compatibility (no preserve file = complete cleaning)
- Add comprehensive documentation in how-it-works.md
- Include example preserve file (.qsgen3_preserve.example)
- Remove legacy images directory and update documentation
- Fix workflow documentation formatting issues

.qsgen3_preserve.example

@@ -0,0 +1,40 @@
+# qsgen3 Preserve File
+# 
+# This file specifies patterns for files that should be preserved during
+# output directory cleaning. This is useful for keeping shared articles
+# or other content that should remain accessible even after title changes.
+#
+# Format:
+# - One pattern per line
+# - Supports shell glob patterns (*, ?, [])
+# - Lines starting with # are comments
+# - Empty lines are ignored
+# - Patterns are relative to the output directory
+#
+# Examples:
+
+# Preserve specific files by exact name
+# posts/my-important-shared-article.html
+# about.html
+
+# Preserve files by pattern
+# posts/legacy-*.html
+# *.pdf
+
+# Preserve entire directories
+# archive/*
+# downloads/*
+
+# Preserve files with specific extensions in certain directories
+# posts/*.html
+# docs/*.pdf
+
+# Example: Preserve a specific shared article
+# posts/how-to-setup-qsgen3.html
+
+# Example: Preserve all files in a legacy directory
+# legacy/*
+
+# Example: Preserve important documents
+# important-*.html
+# *.pdf