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.

2012/omoikane/index.html

@@ -523,6 +523,10 @@ combinations:</p>
 </ul>
 <p>Note that on MingW, <code>stdin</code> and <code>stdout</code> are not opened in binary mode by
 default, this means <code>Nyaruko</code> may not faithfully encode files on MingW.</p>
+<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="extra-files">Extra files</h3>
 <p><a href="obfuscation.html">obfuscation.html</a> - Contains full recording of how the code went from
 blank state to an obfuscated program, gzipped to fit under 1MB. Requires