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/endoh1/index.html

@@ -518,13 +518,13 @@ Analyze the code.</p>
 <p>There are mines in a cell whose “priorities-of-each-cell” is less than
 “number-of-mines”. (But the mine in the first-clicked cell is removed.)</p>
 <h3 id="bug-fixes-and-credits">Bug fixes and Credits</h3>
-<p>The judges pointed out a bug of the original version that I submitted
-(<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/endoh1/prog.alt.c">prog.alt.c</a>). Because this program does not provide a feature to
+<p>The judges pointed out a bug of the original version that I submitted,
+<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/endoh1/prog.alt.c">prog.alt.c</a>. Because this program does not provide a feature to
 flag unprobed cells manually, we can do nothing if the number of unprobed cells
 are equal to the number of mines remaining; <a href="unwinnable.txt">unwinnable.txt</a> is
 an example for such a case.</p>
 <p>Thus, I implemented an additional rule to flag all unprobed cells in such a case
-into the final version (<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/endoh1/prog.c">prog.c</a>).</p>
+into the final version: <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/endoh1/prog.c">prog.c</a>.</p>
 <p><a href="../../authors.html#Ilya_Kurdyukov">Ilya Kurdyukov</a> also pointed out a warning
 “ignoring return value of ‘fscanf’” only when <code>-O3</code> is used. So I changed the
 code a little to pretend to use the return value of <code>fscanf(3)</code>.</p>