add hack-a-round for language specified markdown code blocks

When language is specified in a markdown code block such as:

````
```c
    printf("Hello, world!\n");
```
````

is used, the resulting HTML as produced by `pandoc(1)` combined
with our revised `ioccc.css` is not intent the code block well.

A temporary "_hack-a-round_" is to turn the "markdown code language
specifier" into an HTML comment.  For example:

````
```<!---c-->
    printf("Hello, world!\n");
```
````

This commits turns off existing markdown code language specifiers
for now, while preserving our ability to restore them in the future
when a better  "_hack-a-round_" to the "markdown code language
specifier" problem is found.

The HTML impact if this "_hack-a-round_" is to produce HTML that
looks like this:

```
<pre class="&lt;!---c--&gt;"><code> .. </code></pre>
```

We might be able to create a complex set of classes for each markdown
code language (there won't be very many) in `ioccc.css`, or we might
find a more general CSS (or maybe `pandoc(1)`?) solution.

For now, this hack-a-round will have to do.

1990/theorem/index.html

@@ -141,13 +141,13 @@ Location: <a href="../../location.html#US">US</a> - <em>United States of America
 <!-- START: this line starts content for HTML phase 21 by: bin/pandoc-wrapper.sh via bin/md2html.sh -->
 
 <h2 id="to-build">To build:</h2>
-<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>    <span class="fu">make</span> all</span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    make all</code></pre>
 <h3 id="bugs-and-misfeatures">Bugs and (Mis)features:</h3>
 <p>The current status of this entry is:</p>
 <pre><code>    STATUS: INABIAF - please **DO NOT** fix</code></pre>
 <p>For more detailed information see <a href="../../bugs.html#1990_theorem">1990 theorem bugs</a>.</p>
 <h2 id="to-use">To use:</h2>
-<div class="sourceCode" id="cb3"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./theorem</span> expression x1 x2 h y1</span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    ./theorem expression x1 x2 h y1</code></pre>
 <p>where:</p>
 <pre><code>    expression - function f(x,y)  (see below)
     x1 - start of interval
@@ -155,7 +155,7 @@ Location: <a href="../../location.html#US">US</a> - <em>United States of America
     h - step size
     y1 - initial value  (y(x1) == y1)</code></pre>
 <h2 id="try">Try:</h2>
-<div class="sourceCode" id="cb5"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./try.sh</span></span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    ./try.sh</code></pre>
 <h2 id="judges-remarks">Judges’ remarks:</h2>
 <p>The program’s source implements four functions, all from the
 same source file!</p>
@@ -169,26 +169,26 @@ operators:</p>
 integer powers. Also note that all expressions are evaluated strictly
 left to right. (i.e., parenthesis aren’t supported).</p>
 <p>Try running the program with the following args:</p>
-<div class="sourceCode" id="cb7"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./theorem</span> y 0 1 0.1 1</span>
-<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./theorem</span> 1/x 1 2 0.1 0</span>
-<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./theorem</span> <span class="st">&#39;x^2/y+x&#39;</span> 0 1 0.1 6</span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    ./theorem y 0 1 0.1 1
+    ./theorem 1/x 1 2 0.1 0
+    ./theorem &#39;x^2/y+x&#39; 0 1 0.1 6</code></pre>
 <p>But wait, there is more! You also get, free of charge, a
 reversing filter! Try:</p>
-<div class="sourceCode" id="cb8"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./theorem</span> <span class="at">-r</span> 0 0 0 0 <span class="op">&lt;</span> theorem.c <span class="op">&gt;</span> sorter.c</span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    ./theorem -r 0 0 0 0 &lt; theorem.c &gt; sorter.c</code></pre>
 <p>Still not impressed? The author throws in for free, a
 sort program! Try:</p>
-<div class="sourceCode" id="cb9"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a>    <span class="fu">make</span> sorter</span>
-<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a>    <span class="fu">ls</span> <span class="kw">|</span> <span class="ex">./sorter</span></span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    make sorter
+    ls | ./sorter</code></pre>
 <p>This program is safe for home use as well. The author has
 included a safety feature in case you misplace the original
 program source:</p>
-<div class="sourceCode" id="cb10"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./sorter</span> <span class="at">-r</span> 0 0 0 0 <span class="op">&lt;</span> sorter.c <span class="op">&gt;</span> theorem_bkp.c</span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    ./sorter -r 0 0 0 0 &lt; sorter.c &gt; theorem_bkp.c</code></pre>
 <p>And finally, as a special offer to users of this entry,
 the author provides a Fibonacci sequence generator! Try:</p>
-<div class="sourceCode" id="cb11"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./sorter</span> 0 0 0 0 <span class="op">&lt;</span> theorem.c <span class="op">&gt;</span> fibonacci.c</span>
-<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a>    <span class="fu">make</span> fibonacci</span>
-<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./fibonacci</span> 1 1</span>
-<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a>    <span class="ex">./fibonacci</span> 2 1</span></code></pre></div>
+<pre class="&lt;!---sh--&gt;"><code>    ./sorter 0 0 0 0 &lt; theorem.c &gt; fibonacci.c
+    make fibonacci
+    ./fibonacci 1 1
+    ./fibonacci 2 1</code></pre>
 <p>NOTE: if the two args passed to <code>fibonacci</code> and <code>theorem_bkp</code> add to 0 it will
 print 0 over and over again in an infinite loop. In this case it should not be
 fixed (another condition where this occurred was fixed in 2023 as it affected