add markdown.md and markdown.html

Add `markdown.md` which generates `markdown.html`
to document IOCCC markdown best practices.

Changed `bin/gen-top-html.sh` VERSION from to "1.4.1 2024-04-16"
to "1.4.2 2024-04-21".

Added FAQ 0.6 about IOCCC mark requirements.

Run `make gen_top_html timestamp` to perform the above.

inc/md2html.cfg

@@ -227,6 +227,22 @@ location.md
 	-D
 	./
 
+markdown.md
+	-H
+	navbar=empty
+	-o
+	bin/subst.default.sh
+	-s
+	TITLE=The International Obfuscated C Code Contest
+	-s
+	DESCRIPTION=IOCCC markdown
+	-s
+	KEYWORDS=IOCCC, markdown
+	-s
+	HEADER_2=IOCCC markdown best practices
+	-D
+	./
+
 news.md
 	-H
 	navbar=empty