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.

2019/poikola/index.html

@@ -521,7 +521,11 @@ prime</a> is missing from output.</p>
 achieve exactly the same output from different optimization levels and
 compilers. Small change in code can produce really unexpected results, i.e. the
 compiler can skip a few expressions or statements without obvious reason.</p>
-<h3 id="major-spoilers">Major spoilers</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="how-this-entry-works">How this entry works:</h3>
 <p>It is commonly believed that <a href="http://www.cadaeic.net/calendar.htm">Mike Keith’s
 algorithm</a> published in <em>Journal of
 Recreational Mathematics</em>, Vol. 22, No. 4, 1990, p. 280, is the shortest way to