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.

sitemap.xml

@@ -19098,7 +19098,7 @@
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/bin/gen-top-html.sh</loc>
-    <lastmod>2024-04-17T01:29:11+00:00</lastmod>
+    <lastmod>2024-04-21T09:25:57+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/bin/gen-year-index.sh</loc>
@@ -19222,11 +19222,11 @@
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/faq.html</loc>
-    <lastmod>2024-04-20T20:35:51+00:00</lastmod>
+    <lastmod>2024-04-21T09:51:32+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/faq.md</loc>
-    <lastmod>2024-04-20T20:35:51+00:00</lastmod>
+    <lastmod>2024-04-21T09:47:11+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/.gitignore</loc>
@@ -19314,7 +19314,7 @@
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/inc/md2html.cfg</loc>
-    <lastmod>2024-04-21T05:39:44+00:00</lastmod>
+    <lastmod>2024-04-21T09:28:05+00:00</lastmod>
 </url>
 <url>
     <loc>https://ioccc-src.github.io/temp-test-ioccc/inc/navbar.default.html</loc>