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.

2013/cable3/index.html

@@ -547,7 +547,7 @@ the <code>fd.img</code> provided, which is a FreeDOS boot disk.</p>
 <p>Before running the emulator on a Unix-type system, <code>stty</code> needs to be used to
 put the keyboard into raw mode (and afterwards it needs to be put back to
 cooked). So, run the emulator using something like this script (provided as
-<a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/cable3/cable3.sh">cable3.sh</a>):</p>
+the <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2013/cable3/cable3.sh">cable3.sh</a> file):</p>
 <pre><code>    stty cbreak raw -echo min 0
     ./cable3 bios fd.img
     stty cooked echo</code></pre>
@@ -674,7 +674,7 @@ shutting down the terminal.</p>
 disks (18 sectors per track, 2 heads) and 720KB disks (9 sectors per track, 2
 heads).</p>
 <p>If you want to install your own software from floppy images (downloaded from
-e.g. <a href="https://vetusware.com">Vetusware</a>), the easiest way to “change disks” is
+e.g., vetusware.com at one time), the easiest way to “change disks” is
 to copy each disk image in turn over the floppy image file you specify on the
 command line. Don’t forget to put your original boot disk back at the end!</p>
 <h3 id="hard-disk-support">Hard disk support</h3>