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.

2018/index.html

@@ -415,8 +415,8 @@ of the <a href="poikola/index.html">Most stellar - 2018/poikola</a>.</p>
 <p>The <a href="endoh2/index.html">Best use of python - 2018/endoh2</a> is a not what you might think it is.
 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="1989/yang/index.html">1989/yang</a>, …</p>
+<p>There are also nods to entries of the years <a href="../2000/bellard/index.html">2000/bellard</a>,
+<a href="../1989/yang/index.html">1989/yang</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>The number of entries that would have made it into the final rounds