add notices to those who wish for a greater challenge

We add the following level-3 header to help people identify text
they may wish to skip if they wish for a greater challenge:

```
```

After that notice we add:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.
```

And sometimes follow with:

```
```

Or sometimes follow with:

```
```

Where the alt code impacts a "greater challenge", we add instead:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program without looking at the Alternate code.

If you get stuck, come back and look at the Alternate code.
```

And where a given command impacts a "greater challenge", we add instead:

```
**If you want a greater challenge, don't read any further**:
just try to understand the program without running the command below.

If you get stuck, come back and run the following command:
```

Perform `make www` to perform the above and test under macOS.

1994/horton/index.html

@@ -429,7 +429,11 @@ newsletter and a PDF file of just the pages cited.</p>
 <p>Run it with any 4 numeric arguments, e.g. <code>./horton 3 2 1 0</code>. Play with the
 numbers to get a pleasing result, or use it for your high school algebra
 class.</p>
-<h3 id="spoiler">SPOILER:</h3>
+<h3 id="notice-to-those-who-wish-for-a-greater-challenge">NOTICE to those who wish for a greater challenge</h3>
+<p><strong>If you want a greater challenge, don’t read any further</strong>:
+just try to understand the program via the source.</p>
+<p>If you get stuck, come back and read below for additional hints and information.</p>
+<h3 id="what-this-entry-does">What this entry does:</h3>
 <p>As should be obvious from 20 feet away, the program is a cubic plotter.
 It plots against certain artists, splattering their cubes with graphs
 of their cubic equations. Those dastardly arguments are mere coefficients,
@@ -439,6 +443,7 @@ It’s easily modified to graph <em>any</em> 96x160 bitmap on a dumb terminal.</
 <p>See the <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1994/horton/gtface.c">gtface</a> program for another example. Try:</p>
 <pre><code>    make gtface
     ./gtface &lt; gtface.data</code></pre>
+<h3 id="how-this-entry-works">How this entry works:</h3>
 <p>For more information, see my article</p>
 <p><a href="login_sept92-pp28-31.pdf">Using GraphTab; USENIX ;login: Sept/Oct 1992, pages 28-31</a>.</p>
 <pre><code>          MMMM,M,MM&quot;MM,,,