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.

1994/tvr/index.html

@@ -434,9 +434,9 @@ fix it without breaking something else?</p>
 <h3 id="interactive-fractals-for-x-window-system">Interactive Fractals for X Window System</h3>
 <p>This program contains four pairs of fractals/effects and three methods of
 animating them. There are two windows for a pair of fractals, one of which
-contains static fractal (e.g. <a href="https://en.wikipedia.org/wiki/Mandelbrot_set">Mandelbrot
-set</a>) and the other an animating
-one (e.g. <a href="https://en.wikipedia.org/wiki/Julia_set">Julia set</a>) which is derived
+contains static fractal (e.g., <a href="https://en.wikipedia.org/wiki/Mandelbrot_set">Mandelbrot</a> set)
+and the other an animating
+one (e.g., <a href="https://en.wikipedia.org/wiki/Julia_set">Julia</a> set) which is derived
 from the same formula as the static one. Only one pair of fractals can be
 displayed in one process.</p>
 <p>To animate the fractal, you must move your mouse on the static