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.

2018/yang/README.md

@@ -32,7 +32,7 @@ this program left and recompiling will reveal other tools including a right rota
 and a shift program.
 
 Strange things happen [when the world is upside
-down](https://en.wikipedia.org/wiki/Wacky_Wednesday_(book))! It is entirely possible
+down](https://en.wikipedia.org/wiki/Wacky_Wednesday_(book))! It is entirely possible
 that this remark is completely misleading.
 
 What exactly does the shift program do?
@@ -123,7 +123,7 @@ the 10th final string from [prog.c](%%REPO_URL%%/2018/yang/prog.c).  The intent
 wooden puzzle boxes that can be opened by pushing and shifting various
 well-concealed seams.  Thus, finding the correct sequence of rotates
 and shifts is left as an exercise to the reader (but if you are really
-lazy, just read the [Makefile](%%REPO_URL%%/2018/yang/Makefile)).
+lazy, just read the [Makefile](%%REPO_URL%%/2018/yang/Makefile).
 
 Source code for all the tools needed to solve this puzzle are embedded
 in [prog.c](%%REPO_URL%%/2018/yang/prog.c), all you need is a C compiler.

2018/yang/index.html

@@ -469,7 +469,7 @@ the 10th final string from <a href="https://github.com/ioccc-src/temp-test-ioccc
 wooden puzzle boxes that can be opened by pushing and shifting various
 well-concealed seams. Thus, finding the correct sequence of rotates
 and shifts is left as an exercise to the reader (but if you are really
-lazy, just read the <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2018/yang/Makefile">Makefile</a>).</p>
+lazy, just read the <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2018/yang/Makefile">Makefile</a>.</p>
 <p>Source code for all the tools needed to solve this puzzle are embedded
 in <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2018/yang/prog.c">prog.c</a>, all you need is a C compiler.</p>
 <h3 id="features">Features</h3>