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.

2018/index.html

@@ -412,7 +412,7 @@ It is something completely different.</p>
 <p>The <a href="ciura/index.html">Most likely to be awarded - 2018/ciura</a> has an amazing vocabulary!</p>
 <p>There are also nods to entries of the years <a href="../2000/bellard/index.html">2000/bellard</a>,
 <a href="../2015/yang/index.html">2015/yang</a>, …</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>
 <h3 id="historical-note">Historical note:</h3>
 <p>In 2023-2024 the IOCCC website was drastically changed with the same look for
 every year and this changed the styling as well. But in 2018 different years had