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.

2015/dogon/index.html

@@ -527,6 +527,10 @@ in a portable way without ever using any SSE/AVX or other non-portable compiler
 inline functions! The irony though, is that when compiling the program on modern
 X86 architectures indeed SSE/AVX is used for the long longs. Following is some
 obfuscation information, So if you want to play tough, stop reading at this point.</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="portable-simd-programming">Portable SIMD programming:</h3>
 <p>So how is this done? Quite simply by using the whole <code>long long</code> arithmetic’s
 capability. I think the C99 standard assures us that <code>long long</code> has at least 64