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.

faq.html

@@ -142,6 +142,7 @@
 <li><a href="#faq0_3">0.3 - May I use a different source or compiled filename than prog.c or prog?</a></li>
 <li><a href="#faq0_4">0.4 - What platform should I assume for my entry?</a></li>
 <li><a href="#faq0_5">0.5 - How may I comment or make a suggestion on IOCCC rules, guidelines and tools?</a></li>
+<li><a href="#faq0_6">0.6 - What are the IOCCC requirements for using markdown?</a></li>
 </ul>
 <h2 id="section-1---history-of-the-ioccc">Section 1 - <a href="#faq1">History of the IOCCC</a></h2>
 <ul>
@@ -458,6 +459,22 @@ discussion</a>.</p>
 <p>As a very last resort and/or if you feel the need to engage in a private
 discussion with the <a href="judges.html">IOCCC judges</a>, then see
 <a href="contact.html">How to contact the IOCCC</a>.</p>
+<div id="faq0_6">
+<div id="markdown">
+<h3 id="faq-0.6---what-are-the-ioccc-requirements-for-using-markdown">FAQ 0.6: - What are the IOCCC requirements for using markdown?</h3>
+</div>
+</div>
+<p>The IOCCC makes extensive use of <a href="https://daringfireball.net/projects/markdown/">markdown</a>.
+For example, we <a href="faq.html#submit">submitting to the IOCCC</a>, we have people
+to submit remarks about entry in markdown format. Every
+<a href="years.html">winning IOCCC entry</a> uses a <code>README.md</code> markdown file
+as the basis for forming the <code>index.html</code> web page for that entry.
+All generated HTML pages on the <a href="https://www.ioccc.org/index.html">Official IOCCC web site</a>
+start with some markdown content.</p>
+<p><strong>IMPORTANT</strong>: Please read the <a href="markdown.html">IOCCC markdown best practices</a> guide
+as it lists things you <strong>should not use</strong> in markdown files.</p>
+<p>See the <a href="https://www.markdownguide.org/basic-syntax">markdown syntax</a> guide.
+See also <a href="https://spec.commonmark.org/current/">CommonMark Spec</a>.</p>
 <div id="faq1">
 <h2 id="section-1-history-of-the-ioccc">Section 1: History of the IOCCC</h2>
 </div>