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.

1992/gson/index.html

@@ -487,8 +487,12 @@ eliminated. As AG also refrains from abusing the preprocessor, it
 doesn’t really have much to offer in terms of “surface obfuscation”.
 Instead, it tries to achieve both its speed and its obscurity through a
 careful choice of algorithms. Some of the finer points of those
-algorithms are outlined in the spoiler below.</p>
-<h3 id="how-it-works">How it works:</h3>
+algorithms are outlined the section below.</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="how-this-entry-works">How this entry works:</h3>
 <p>Here follows a description of some of the data structures and
 algorithms used by AG. It is by no means complete, but it may help
 you get an idea about the general principles.</p>