Fix html.sed and related scripts

The file bin/html.sed has been fixed so that it works with
bin/md2html.sh (which has been modified and which is used via
bin/subst.entry-index.sh and output-index-inventory.sh). The script
output-index-inventory.sh was already updated to use html.sed so this
file has not been modified but the md2html.sh and subst.entry-index.sh
scripts have, along with bin/html.sed itself.

Tested make www which rebuilt several index.html files.

Updated bin/README.md as well as bin/index.html.

Testing that all the html files now are correct wrt Nu checker will have
to be done another time, however.

1988/robison/index.html

@@ -406,7 +406,7 @@
   </a>
   <h1>The International Obfuscated C Code Contest</h1>
   <h2>1988/robison - Best abuse of C constructs</h2>
-  <h3>print e in any base using only -- >= and while()</h3>
+  <h3>print e in any base using only -- &gt;= and while()</h3>
 </div>
 
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