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.

2011/richards/README.md

@@ -33,7 +33,7 @@ For more detailed information see [2011 richards bugs](../../bugs.html#2011_rich
 ## Judges' remarks:
 
 Oh no, yet another
-[dc](https://en.wikipedia.org/wiki/Dc_(computer_program))-style calculator? (see
+[dc](https://en.wikipedia.org/wiki/Dc_(computer_program))-style calculator? (see
 [2000/dlowe](../../2000/dlowe/index.html) using an embedded Perl interpreter and deemed `Worst
 Abuse of the Rules`).  Well, this one does it (expression calculation, that is,
 not the rule abuse) in a different way.  The author claims that his calculator
@@ -44,7 +44,7 @@ without attempting to unravel the source?
 ## Author's remarks:
 
 This is an implementation of the classic UNIX
-[dc](https://en.wikipedia.org/wiki/Dc_(computer_program)) command, the
+[dc](https://en.wikipedia.org/wiki/Dc_(computer_program)) command, the
 [reverse Polish](https://en.wikipedia.org/wiki/Reverse_Polish_notation)
 calculator. Actually it's a pretty bad implementation, since it only implements
 32-bit integers (well, `sizeof(int)`-sized integers). It only implements the