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.

bin/html.sed

@@ -14,6 +14,10 @@
 # to their respective html entity, for instance '<', so that html lints do
 # not complain.
 #
+# For a list of the other html entities, should others be needed, see:
+#
+#       https://www.w3schools.com/html/html_entities.asp
+#
 # usage:
 #
 #	cmd | sed -f bin/html.sed
@@ -27,6 +31,6 @@
 # IMPORTANT NOTE: the '&' substitution MUST come first! This is because the
 # other substitutions have '&'s in them and if the '&' came after it would
 # corrupt the '&'s in the string.
-s/&/\&/g
-s/</\&lt;/g
-s/>/\&gt;/g
+s/&/\\\&amp;/g
+s/</\\\&lt;/g
+s/>/\\\&gt;/g