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.

bin/gen-top-html.sh

@@ -88,7 +88,7 @@ TOP_MD_SET+=("faq")
 
 # set variables referenced in the usage message
 #
-export VERSION="1.4.1 2024-04-16"
+export VERSION="1.4.2 2024-04-21"
 NAME=$(basename "$0")
 export NAME
 export V_FLAG=0
@@ -124,6 +124,7 @@ TOP_MD_SET+=("inc/README")
 TOP_MD_SET+=("index")
 TOP_MD_SET+=("judges")
 TOP_MD_SET+=("license")
+TOP_MD_SET+=("markdown")
 TOP_MD_SET+=("news")
 TOP_MD_SET+=("next/README")
 TOP_MD_SET+=("next/guidelines")