fix more broken markdown links

We fix more markdown links in markdown files that refer to files
internally to the web site.

Added tmp tools to find and print various markdown links:

* `tmp/md-links.sh` - print all markdown links
* `tmp/md-token-links.sh` - print all markdown links that start with a **%%TOKEN%%**
* `tmp/md-local-links.sh` - print all markdown links to a local file
* `tmp/md-link-paths.sh` - print local file paths referenced in markdown links

Fixed a number of markdown formatting problems with `thanks-for-help.md`.
Fixed markdown links to files in `thanks-for-help.md`.  Improved
heading levels in `thanks-for-help.md`, and made entry sections consistent.

Changed all markdown links that end in "))" to end in a better way.

Explained in `markdown.md` why we are avoiding markdown links that
end in "))".

Ran `make www` to perform the above.

TODO: Fix stuff we broke by mistake doing the above.

TODO: Fix more broken markdown links.

2001/dgbeards/README.md

@@ -162,7 +162,7 @@ program would probably be rated somewhere around 1700-1900.  It is better
 than most players, but will get destroyed by experienced human opponents.
 
 This program searches to a fixed depth of 3
-[ply](https://en.wikipedia.org/wiki/Ply_(game_theory)).  Because it does not
+[ply](https://en.wikipedia.org/wiki/Ply_(game_theory)).  Because it does not
 search very deep, it performs very poorly in quiet strategic positions.
 In such positions, it will often just shuffle pieces around aimlessly
 until a tactical exchange presents itself.