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/index.html

@@ -415,7 +415,7 @@ are rather similar to previous entries</p>
 </blockquote>
 <p>because the entry <a href="dogon/index.html">2019/dogon</a> appropriately pays homage
 to past entries without blatantly reusing their code.</p>
-<p>…We’ll stop spouting spoilers now. Have fun exploring all the entries!</p>
+<p>…We’ll stop spouting too many details now. Have fun exploring all the entries!</p>
 <h2 id="remarks-on-some-of-submissions-that-did-not-win">Remarks on some of submissions that did not win</h2>
 <p>Some entries tried to get around the size limit by submitting a data
 file that was really C code and then blindly including code in prog.c.</p>