improve markdown

Fixed a number of whitespace, HTML comments, intending,
and markdown problems.

Clarified the <!---language--> syntax.

Made a number of textual changes to be more consistent.

Replaced emoji with HTML symbols.

Misc cleanup.

Ran `make markdown sitemap` to perform the above.

sitemap.xml

@@ -16810,11 +16810,11 @@
 </url>
 <url>
     <loc>https://www.ioccc.org/markdown.html</loc>
-    <lastmod>2025-11-28T05:02:34+00:00</lastmod>
+    <lastmod>2025-11-28T16:46:49+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/markdown.md</loc>
-    <lastmod>2025-11-28T04:59:17+00:00</lastmod>
+    <lastmod>2025-11-28T16:46:44+00:00</lastmod>
 </url>
 <url>
     <loc>https://www.ioccc.org/news.html</loc>