add experimental rules to build web site

We add to the top level Makefile, some initial rules that
will facilitate the building of HTML content.

The `bin/md2html.sh` tool as been updated to output HTML comments
about the output during HTML phases 20, 21 and 22.

All entry `index.html` files were updated with some additional HTML
comments as a result of the above.  The top level `location.html`,
`years.html` and `authors.html` have had HTML comments added also
a result.

Sorry (tm Canada 🇨🇦) for the mass update of entry `index.html` files.
Hopefully the times when all of them (often) need to be updated
will soon draw to a close.

years.html

@@ -100,6 +100,8 @@
     <a name="content"></a>
 
 <!-- END: this line ends content from: inc/before-content.default.html -->
+<!-- START: this line starts content for HTML phase 21 by: bin/pandoc-wrapper.sh via bin/md2html.sh -->
+
 <h1 id="ioccc-winning-entries-by-year">IOCCC Winning Entries by
 Year</h1>
 <h2 id="all-ioccc-years">All IOCCC years</h2>
@@ -858,6 +860,8 @@ Dishonorable mention</li>
 Prize</li>
 </ul>
 <p> <strong>Jump to:</strong> <a href="#">top</a></p>
+
+<!-- END: this line ends content for HTML phase 21 by: bin/pandoc-wrapper.sh via bin/md2html.sh -->
 <!-- START: this line starts content from: inc/after-content.default.html -->
 
 <!-- END: this line ends content from: inc/after-content.default.html -->