feat: add XSLT to convert to Markdown

[PeterMosmans]

These XSLT files convert a report into Markdown format. Note that the Makefile hasn't been touched, as that's a pretty old vanilla one.
Example to create a target/report.md:

# Create Markdown from XML
$(TARGET)/%.md: source/%.xml $(SOURCE) $(TARGET)
	@$(JAVA) -jar saxon.jar \
	net.sf.saxon.Transform \
	     -s:source/report.xml \
	     -xsl:"xslt/generate_md_$*.xsl" \
	     -xi > "$@"