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.

2012/tromp/index.html

@@ -435,8 +435,8 @@ the remainder of input (translated to a lazy list of
 representation in lambda calculus), and translates the evaluated result back
 into a stream of bits to be output.</p>
 <p>Lambda is encoded as 00, application as 01, and the variable bound by the <code>n</code>th
-enclosing lambda (denoted <code>n</code> in so-called <a href="https://en.wikipedia.org/wiki/De_Bruijn_notation">De Bruijn
-notation</a>) as <code>1^{n}0</code>.
+enclosing lambda (denoted <code>n</code> in so-called <a href="https://en.wikipedia.org/wiki/De_Bruijn_notation">De Bruijn</a>
+notation) as <code>1^{n}0</code>.
 That’s all there is to
 <a href="https://tromp.github.io/cl/Binary_lambda_calculus.html">BLC</a>!</p>
 <p>For example the encoding of lambda term <code>S = \x \y \z  (x z) (y z)</code>,
@@ -459,8 +459,8 @@ bit of input, and using characters <code>0</code> and <code>1</code> for output.
     1,2,3,4,8,9: result not in list form</code></pre>
 <p>The size of the term space is fixed at compile time with <code>-DA</code>.</p>
 <h3 id="a-half-byte-cat">A half byte <code>cat</code></h3>
-<p>The shortest (closed) lambda calculus term is <code>\x x (\ 1</code> in <a href="https://en.wikipedia.org/wiki/De_Bruijn_notation">De Bruijn
-notation</a>) which is the
+<p>The shortest (closed) lambda calculus term is <code>\x x (\ 1</code> in
+<a href="https://en.wikipedia.org/wiki/De_Bruijn_notation">De Bruijn</a> notation which is the
 identity function. When its encoding <code>0010</code> is fed into the universal machine,
 it will simply copy the input to the output (well, not that simply, since each
 byte is smashed to bits and rebuilt from scratch). Voila: a half byte cat:</p>