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/birken/index.html

@@ -432,7 +432,7 @@ circuits)? This program is my tribute to the early computer pioneers like
 tool designed to introduce students to digital circuit theory using technology
 that was considered state of the art in the 1940s! To enable you to fully
 understand the capabilities of this circuit simulator, I have included a 55 page
-manual (<a href="info.pdf">info.pdf</a>) in the info portion of the entry submission.</p>
+manual, <a href="info.pdf">info.pdf</a>, in the info portion of the entry submission.</p>
 <p>The primary obfuscation is the data structure chosen for parsing the
 input file and for executing the simulation. Oh man… coding that
 data structure gave me such a bad headache. But, I figure, who needs