update markdown version

Changed `markdown.md` VERSION from "28.1 2025-02-02"
to "28.2 2025-04-28".

Updated sitemap.

Tested the above via `make www gen_status gen_sitemap timestamp` on macOS.

markdown.html

@@ -431,7 +431,7 @@
 <h2 id="ioccc-markdown-guidelines-version">IOCCC Markdown guidelines version</h2>
 </div>
 <p class="leftbar">
-These <a href="markdown.html">IOCCC markdown guidelines</a> are version <strong>28.1 2025-02-02</strong>.
+These <a href="markdown.html">IOCCC markdown guidelines</a> are version <strong>28.2 2025-04-28</strong>.
 </p>
 <p><strong>IMPORTANT</strong>: Be sure to read the <a href="next/rules.html">IOCCC rules</a> and <a href="next/guidelines.html">IOCCC
 guidelines</a>.</p>