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.

2006/meyer/README.md

@@ -89,10 +89,10 @@ include:
 - constructors (invoked by the new operator).
 - new operators (every class gets its own new operator at compile time).
 - virtual method tables (all the virtual method tables have been combined to a
-single [vtable](https://en.wikipedia.org/wiki/Virtual_method_table)).
+single [vtable](https://en.wikipedia.org/wiki/Virtual_method_table) table).
 - runtime method binding.
-- [inheritance](https://en.wikipedia.org/wiki/Inheritance_(object-oriented_programming))
-and [polymorphism](https://en.wikipedia.org/wiki/Polymorphism_(computer_science)).
+- [inheritance](https://en.wikipedia.org/wiki/Inheritance_(object-oriented_programming))
+and [polymorphism](https://en.wikipedia.org/wiki/Polymorphism_(computer_science)).
 
 It is left as an exercise to the reader to find a corresponding
 implementation in a popular object-oriented programming language.