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.

2006/monge/index.html

@@ -468,7 +468,7 @@ separated list of assignments to do once per pixel, and the second a semicolon
 separated list of assignments and conditions that will be executed (and
 conditions checked) for each iteration (up to the maximum). Using the left and
 right mouse buttons you will be able to zoom in and out (a la
-<a href="https://github.com/xaos-project/XaoS">XaoS</a>), the zooming algorithm is a bit
+<a href="https://github.com/xaos-project/XaoS">XaoS</a> project), the zooming algorithm is a bit
 slower than <code>XaoS</code>’s but it should give slightly better results.</p>
 <p>Formulas can use all <code>a-z</code> (lowercase) variables, and the special
 variables <code>c</code> and <code>i</code> are respectively set by default to the