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.

2020/index.html

@@ -405,7 +405,7 @@ IOCCC list!</p>
 <a href="tsoj/index.html">Asteroids - 2020/tsoj</a>, <a href="endoh1/index.html">Minesweeper - 2020/endoh1</a>,
 and <a href="ferguson1/index.html">Snake - 2020/ferguson1</a>.</p>
 <p>An entry, <a href="kurdyukov1/index.html">kurdyukov1</a>, pays homage to previous entry
-<a href="2015/hou/index.html">2015/hou</a>.</p>
+<a href="../2015/hou/index.html">2015/hou</a>.</p>
 <p>…We’ll stop spouting spoilers 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>As a rule, we try to compile the entries on a variety of platforms. Quite a few