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.

2013/endoh3/index.html

@@ -516,7 +516,11 @@ such as <code>C2E2</code>.</p>
 But I do not attach it to protect you from W****r.</p>
 <p>For the same reason, do not post a score that contains only <code>z1092</code>.
 You know, it is <a href="http://en.wikipedia.org/wiki/4%E2%80%B233%E2%80%B3">“the famous song”</a>.</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="how-this-entry-works">How this entry works:</h3>
 <p>Here is a magical expression which I found by brute-force:</p>
 <pre><code>    (c % 32 + 5) * 9 / 5 % 13 + n / 32 * 12 - 22</code></pre>
 <p>Interestingly, it converts an ASCII number of ABC notes