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.

2004/vik2/README.md

@@ -157,7 +157,7 @@ Then run the program to reveal all prime numbers up to 32 (if you used
 
 I wasn't able to compile the full program which calculates [prime
 numbers](https://en.wikipedia.org/wiki/Prime_number) up to 1024 using GNU `cpp`
-(or any other [pre-processor](https://en.wikipedia.org/wiki/C_preprocessor)) so
+(or any other [pre-processor](https://en.wikipedia.org/wiki/C_preprocessor) tool) so
 I had to write my own. Knowing how the program works it was quite easy to reduce
 the compilation time from several hours to about one second. I also added some
 statistics to my home made pre-processor and I found that calculating prime

2004/vik2/index.html

@@ -504,7 +504,7 @@ work. This will take some time….</p>
 <h3 id="verification">Verification</h3>
 <p>I wasn’t able to compile the full program which calculates <a href="https://en.wikipedia.org/wiki/Prime_number">prime
 numbers</a> up to 1024 using GNU <code>cpp</code>
-(or any other <a href="https://en.wikipedia.org/wiki/C_preprocessor">pre-processor</a>) so
+(or any other <a href="https://en.wikipedia.org/wiki/C_preprocessor">pre-processor</a> tool) so
 I had to write my own. Knowing how the program works it was quite easy to reduce
 the compilation time from several hours to about one second. I also added some
 statistics to my home made pre-processor and I found that calculating prime