improve markdown sub-section examples

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>28.3 2025-04-28</strong>.
+These <a href="markdown.html">IOCCC markdown guidelines</a> are version <strong>28.4 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>
@@ -468,14 +468,22 @@ not apply to other markdown files), <strong>PLEASE</strong> start at level three
 to 3 for new sections. For instance:</p>
 <pre><code>    ### Section 0
     ...
-    #### First subsection of section 0
+    #### 1st subsection of section 0
     ...
-    ##### Second subsection of section 0
+    ##### 1st sub-subsection of the 1st subsection of section 0
     ...
-    ###### Third subsection of section 0
+    #### 2nd subsection of section 0
+    ...
+    ##### 1st sub-subsection of the 2nd subsection of section 0
+    ...
+    ##### 2nd sub-subsection of the 2nd subsection of section 0
+    ...
+    ###### 1st sub-sub-subsection of the 2nd sub-subsection of the 2nd subsection of section 0
     ...
     ### Section 1
     ...
+    ### Section 2
+    ...
     Etc.</code></pre>
 <div id="name">
 <div id="anchor-name">