fix even more broken markdown links

Added `bin/find-missing-links.sh` tool to check for
markdown links to missing local files.

Regarding the missing link tool:

```
    Here be extended regular expression magic dragons! :-)
```

Added `make find_missing_links` to the top level Makefile.

Added `make find_missing_links` to `make www` and `make quick_www`.

Ran `make www` to perform the above.

TODO: Fix markdown links to missing local files.

1993/rince/index.html

@@ -430,7 +430,7 @@ changed).</p>
 <p>The author has provided several CABBAGE files for your amusement:</p>
 <ul>
 <li><p><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1993/rince/rince.data1">rince.data1</a> plays
-[Breakout](https://en.wikipedia.org/wiki/Breakout_(video_game). Keys are <code>,</code>
+<a href="https://en.wikipedia.org/wiki/Breakout_(video_game)">Breakout</a>. Keys are <code>,</code>
 for left, <code>/</code> for right, and <code>.</code> for ‘fire the ball’.</p></li>
 <li><p><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1993/rince/rince.data2">rince.data2</a> plays the old <a href="../../authors.html#James_Bonfield">James Bonfield</a>’s
 (<a href="../../1991/rince/index.html">1991/rince</a> game from IOCCC 1991.</p></li>