fix HTML problems related to pandoc and markdown image notation

Improve `make clobber` for `2004/anonymous`.

Instead of using this markdown element to embed an image:

```
![alt text](filename.png "Title")
```

Use a "img" HTML element with "alt", "alt", "width" and "length"
attributes:

```
<img src="filename.png"
 alt="describe the filename.png image for someone who cannot view it"
 width=PIXEL_WIDTH height=PIXEL_HEIGHT>
```

This was performed for:

```
2004/anonymous/index.html
2005/boutines/index.html
2011/goren/index.html
2012/vik/index.html
2013/birken/index.html
2013/cable1/index.html
2013/cable3/index.html
2013/hou/doc/example.html
2013/hou/index.html
2020/ferguson1/chocolate-cake.html
```

In the case of `2013/cable1/index.html` we reduce the displayed
image size by a factor for 2.  There is **NO** need to downsize the image
as this image size reduction produces a clean and sharp looking image.

In the case of `2013/hou/index.html` we reduce the displayed
image size by a factor for 2.  There is **NO** need to downsize the image
as this image size reduction produces a clean and sharp looking image.

In the case of `2020/ferguson1/chocolate-cake.html` we reduce the displayed
image size by a factor for 4.  There is **NO** need to downsize the image
as this image size reduction produces a clean and sharp looking image.

Run `make www` to perform the above.

2013/hou/index.html

@@ -170,18 +170,15 @@ every so often reopen the file, to see how it changes. The author suggests that
 you leave the program running overnight to see what happens. If however you do
 not have all night :-) then the following JPEG files should show you what
 happens with some of the invocations above:</p>
-<figure>
-<img src="luna.jpg" alt="./hou BIG - Luna" />
-<figcaption aria-hidden="true">./hou BIG - Luna</figcaption>
-</figure>
-<figure>
-<img src="old_default.jpg" alt="./hou old_default.scene - Old default" />
-<figcaption aria-hidden="true">./hou old_default.scene - Old default</figcaption>
-</figure>
-<figure>
-<img src="otherroom.jpg" alt="./hou otherroom.scene - Other room" />
-<figcaption aria-hidden="true">./hou otherroom.scene - Other room</figcaption>
-</figure>
+<p><img src="luna.jpg"
+ alt="artificial image showing a room lit by light from a window"
+ width=640 height=480></p>
+<p><img src="old_default.jpg"
+ alt="artificial image showing IOCCC 2013 in a room with stars on the floor and double helix in the backgrond"
+ width=640 height=480></p>
+<p><img src="otherroom.jpg"
+ alt="artificial image the background double helix"
+ width=320 height=240></p>
 <p>This program does not terminate by itself: you must kill <code>hou</code> (but not Qiming
 Hou :-) ) in order to end the program.</p>
 <h2 id="judges-remarks">Judges’ remarks:</h2>