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/leo/index.html

@@ -414,7 +414,7 @@ for instructions of how to play.</p>
 International Internet community.</p>
 <h2 id="authors-remarks">Author’s remarks:</h2>
 <p>This program plays
-[Mastermind](https://en.wikipedia.org/wiki/Mastermind_(board_game), if you call
+<a href="https://en.wikipedia.org/wiki/Mastermind_(board_game)">Mastermind</a>, if you call
 it by a name ending by ‘d’, otherwise it <em>solves</em> Mastermind. Call it without
 parameters to use 6 digits (colors), or indicate the number of possible digits
 (you cannot change the number of positions so easily).</p>