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.

2019/README.md

@@ -45,7 +45,7 @@ These nods to not run afoul of the guideline:
     are rather similar to previous entries
 ```
 
-because the entry [2019/dogon](2019/dogon/index.html) appropriately pays homage
+because the entry [2019/dogon](dogon/index.html) appropriately pays homage
 to past entries without blatantly reusing their code.
 
 ...We'll stop spouting spoilers now. Have fun exploring all the entries!