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.

2020/ferguson2/README.md

@@ -288,7 +288,7 @@ After that you can input the string and it'll go from there.
 BTW: There's a much more entertaining (and delicious) challenge or exercise in
 [recode.html][] (if you're reading this on GitHub check the
 [recode.html](recode.html) file instead); this involves [chocolate-cake.html][] (if
-on GitHub see [here](chocolate-cake.html)) though one might need a different kind
+on GitHub, see [here](chocolate-cake.html), though one might need a different kind
 of exercise after taking up the challenge! :-) These however show the general
 program as well as how to use the two winning entries of the [Morse
 code](https://en.wikipedia.org/wiki/Morse_code) that I referred to earlier:
@@ -423,7 +423,7 @@ it prompts you for the settings
 settings](https://en.wikipedia.org/wiki/Enigma_rotor_details#Ring_setting) and
 positions,
 [reflector](https://www.ciphermachinesandcryptology.com/en/enigmatech.htm#Reflector) and
-[plugboard pairs](https://www.cryptomuseum.com/crypto/enigma/i/sb.htm)),
+[plugboard pairs](https://www.cryptomuseum.com/crypto/enigma/i/sb.htm),
 validating input as you go.
 
 After that it prompts for text that you can pass to the Enigma simulator;
@@ -775,7 +775,7 @@ of the Wehrmacht rather than the Wehrmacht and the Kriegsmarine.
 
 [Enigma Message Procedures][]: The [Heer][],
 [Luftwaffe](https://en.wikipedia.org/wiki/Luftwaffe) and
-[Kriegsmarine](https://en.wikipedia.org/wiki/Kriegsmarine))
+[Kriegsmarine](https://en.wikipedia.org/wiki/Kriegsmarine)
 [Enigma](https://en.wikipedia.org/wiki/Enigma_machine)
 procedures.