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.

1992/vern/index.html

@@ -499,8 +499,8 @@ by comparing the function pointer with the function itself).</p>
 <p>Fourth, there are no independent routines for generating potential moves.
 Instead, the program generates its moves by brute force: for every piece it
 controls on the board, it attempts to move it to every square on the board.
-Those moves that are legal it then explores to see their effects (using
-<a href="https://en.wikipedia.org/wiki/Alpha-beta_pruning">alpha-beta search</a>). This
+Those moves that are legal it then explores to see their effects, using
+<a href="https://en.wikipedia.org/wiki/Alpha-beta_pruning">alpha-beta search</a>. This
 tactic somewhat obfuscates the algorithm used by the program.</p>
 <p>Finally, there are three key constants that occur throughout the program: <code>64</code>,
 <code>8</code>, and <code>3</code> (<code>lg 8</code>). Rather than making these available at compile-time,