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/README.md

@@ -33,7 +33,7 @@ For some reason, this year's set of winning entries contains three nostalgic gam
 and [Snake - 2020/ferguson1](ferguson1/index.html).
 
 An entry, [kurdyukov1](kurdyukov1/index.html), pays homage to previous entry
-[2015/hou](2015/hou/index.html).
+[2015/hou](../2015/hou/index.html).
 
 ...We'll stop spouting spoilers now. Have fun exploring all the entries!